Why is this equality wrong? Can't find a proof












1












$begingroup$


I can't understant why these two formulas are not equal, I am certain they are but I cannot find a proof for my claim:
$$sum_{i=0}^{x-1} 2^{i} = 2^x-1$$



Edit: They are equal, thanks for the help










share|cite|improve this question









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Jack Möller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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  • 3




    $begingroup$
    Prove by induction on $xgeq 1$.
    $endgroup$
    – Wuestenfux
    1 hour ago










  • $begingroup$
    Did you really mean to say "(...) are not equal, I am certain they are"?
    $endgroup$
    – StackTD
    1 hour ago










  • $begingroup$
    Multiply the left hand side by $(2-1)$ using the distributive law, simplify.
    $endgroup$
    – Ned
    1 hour ago










  • $begingroup$
    If you are familiar with geometric progressions, you could also use $sum_limits{i = 0}^{x-1} u_1r^i = frac{u_1left(1-r^xright)}{1-r}$.
    $endgroup$
    – KM101
    1 hour ago


















1












$begingroup$


I can't understant why these two formulas are not equal, I am certain they are but I cannot find a proof for my claim:
$$sum_{i=0}^{x-1} 2^{i} = 2^x-1$$



Edit: They are equal, thanks for the help










share|cite|improve this question









New contributor




Jack Möller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







$endgroup$








  • 3




    $begingroup$
    Prove by induction on $xgeq 1$.
    $endgroup$
    – Wuestenfux
    1 hour ago










  • $begingroup$
    Did you really mean to say "(...) are not equal, I am certain they are"?
    $endgroup$
    – StackTD
    1 hour ago










  • $begingroup$
    Multiply the left hand side by $(2-1)$ using the distributive law, simplify.
    $endgroup$
    – Ned
    1 hour ago










  • $begingroup$
    If you are familiar with geometric progressions, you could also use $sum_limits{i = 0}^{x-1} u_1r^i = frac{u_1left(1-r^xright)}{1-r}$.
    $endgroup$
    – KM101
    1 hour ago
















1












1








1





$begingroup$


I can't understant why these two formulas are not equal, I am certain they are but I cannot find a proof for my claim:
$$sum_{i=0}^{x-1} 2^{i} = 2^x-1$$



Edit: They are equal, thanks for the help










share|cite|improve this question









New contributor




Jack Möller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







$endgroup$




I can't understant why these two formulas are not equal, I am certain they are but I cannot find a proof for my claim:
$$sum_{i=0}^{x-1} 2^{i} = 2^x-1$$



Edit: They are equal, thanks for the help







linear-algebra summation proof-explanation equivalence-relations






share|cite|improve this question









New contributor




Jack Möller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











share|cite|improve this question









New contributor




Jack Möller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|cite|improve this question




share|cite|improve this question








edited 1 hour ago







Jack Möller













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asked 1 hour ago









Jack MöllerJack Möller

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62




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New contributor





Jack Möller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






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Check out our Code of Conduct.








  • 3




    $begingroup$
    Prove by induction on $xgeq 1$.
    $endgroup$
    – Wuestenfux
    1 hour ago










  • $begingroup$
    Did you really mean to say "(...) are not equal, I am certain they are"?
    $endgroup$
    – StackTD
    1 hour ago










  • $begingroup$
    Multiply the left hand side by $(2-1)$ using the distributive law, simplify.
    $endgroup$
    – Ned
    1 hour ago










  • $begingroup$
    If you are familiar with geometric progressions, you could also use $sum_limits{i = 0}^{x-1} u_1r^i = frac{u_1left(1-r^xright)}{1-r}$.
    $endgroup$
    – KM101
    1 hour ago
















  • 3




    $begingroup$
    Prove by induction on $xgeq 1$.
    $endgroup$
    – Wuestenfux
    1 hour ago










  • $begingroup$
    Did you really mean to say "(...) are not equal, I am certain they are"?
    $endgroup$
    – StackTD
    1 hour ago










  • $begingroup$
    Multiply the left hand side by $(2-1)$ using the distributive law, simplify.
    $endgroup$
    – Ned
    1 hour ago










  • $begingroup$
    If you are familiar with geometric progressions, you could also use $sum_limits{i = 0}^{x-1} u_1r^i = frac{u_1left(1-r^xright)}{1-r}$.
    $endgroup$
    – KM101
    1 hour ago










3




3




$begingroup$
Prove by induction on $xgeq 1$.
$endgroup$
– Wuestenfux
1 hour ago




$begingroup$
Prove by induction on $xgeq 1$.
$endgroup$
– Wuestenfux
1 hour ago












$begingroup$
Did you really mean to say "(...) are not equal, I am certain they are"?
$endgroup$
– StackTD
1 hour ago




$begingroup$
Did you really mean to say "(...) are not equal, I am certain they are"?
$endgroup$
– StackTD
1 hour ago












$begingroup$
Multiply the left hand side by $(2-1)$ using the distributive law, simplify.
$endgroup$
– Ned
1 hour ago




$begingroup$
Multiply the left hand side by $(2-1)$ using the distributive law, simplify.
$endgroup$
– Ned
1 hour ago












$begingroup$
If you are familiar with geometric progressions, you could also use $sum_limits{i = 0}^{x-1} u_1r^i = frac{u_1left(1-r^xright)}{1-r}$.
$endgroup$
– KM101
1 hour ago






$begingroup$
If you are familiar with geometric progressions, you could also use $sum_limits{i = 0}^{x-1} u_1r^i = frac{u_1left(1-r^xright)}{1-r}$.
$endgroup$
– KM101
1 hour ago












5 Answers
5






active

oldest

votes


















4












$begingroup$

$$
S(x) = sum_{i=0}^{x-1} 2^{i} \
2 S(x) = 2 sum_{i=0}^{x-1} 2^{i} = sum_{i=0}^{x-1} 2^{i+1} = sum_{k=1}^{x} 2^{k} = S(x) + 2^x - 1
$$

so this is your proof that indeed $ S(x) = 2^x - 1 $.






share|cite|improve this answer











$endgroup$





















    3












    $begingroup$

    The easiest way is to notice that
    $$
    sum_{i=0}^{x-1}2^i=sum_{i=0}^{x-1}(2^{i+1}-2^{i})=({2^1}-2^0)+(2^2-2^1)+(2^3-2^2)+dotsb+(2^x-2^{x-1})
    $$

    which is a telescoping series.






    share|cite|improve this answer









    $endgroup$





















      0












      $begingroup$

      Use induction to prove it:




      • Show it is true for $x=1$


      • Assume it is true for $x=k$, this means that you are allowed to use $$sum_{i=0}^{k-1} 2^{i} = 2^k-1$$


      • Then show that it is true for $n=k+1$ by using the above formula.







      share|cite|improve this answer









      $endgroup$





















        0












        $begingroup$

        You can prove this equality with the following method:
        x belongs to N*
        For x=1, 2^0 = 2^1 - 1 = 1
        For x=2, 2^0 + 2^1 = 2^2 - 1 = 3
        .
        .
        .
        Assume that the equality is correct up to (x-1) and demonstrate for the next order (see attached image),



        Proof



        Thus, the equality holds for all x in N*






        share|cite|improve this answer








        New contributor




        Wane Mamadou is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
        Check out our Code of Conduct.






        $endgroup$





















          0












          $begingroup$


          Applying the finite geometric series formula we obtain
          begin{align*}
          sum_{i=0}^{x-1}2^i=frac{2^x-1}{2-1}=2^x-1
          end{align*}







          share|cite|improve this answer









          $endgroup$













            Your Answer





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            5 Answers
            5






            active

            oldest

            votes








            5 Answers
            5






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            4












            $begingroup$

            $$
            S(x) = sum_{i=0}^{x-1} 2^{i} \
            2 S(x) = 2 sum_{i=0}^{x-1} 2^{i} = sum_{i=0}^{x-1} 2^{i+1} = sum_{k=1}^{x} 2^{k} = S(x) + 2^x - 1
            $$

            so this is your proof that indeed $ S(x) = 2^x - 1 $.






            share|cite|improve this answer











            $endgroup$


















              4












              $begingroup$

              $$
              S(x) = sum_{i=0}^{x-1} 2^{i} \
              2 S(x) = 2 sum_{i=0}^{x-1} 2^{i} = sum_{i=0}^{x-1} 2^{i+1} = sum_{k=1}^{x} 2^{k} = S(x) + 2^x - 1
              $$

              so this is your proof that indeed $ S(x) = 2^x - 1 $.






              share|cite|improve this answer











              $endgroup$
















                4












                4








                4





                $begingroup$

                $$
                S(x) = sum_{i=0}^{x-1} 2^{i} \
                2 S(x) = 2 sum_{i=0}^{x-1} 2^{i} = sum_{i=0}^{x-1} 2^{i+1} = sum_{k=1}^{x} 2^{k} = S(x) + 2^x - 1
                $$

                so this is your proof that indeed $ S(x) = 2^x - 1 $.






                share|cite|improve this answer











                $endgroup$



                $$
                S(x) = sum_{i=0}^{x-1} 2^{i} \
                2 S(x) = 2 sum_{i=0}^{x-1} 2^{i} = sum_{i=0}^{x-1} 2^{i+1} = sum_{k=1}^{x} 2^{k} = S(x) + 2^x - 1
                $$

                so this is your proof that indeed $ S(x) = 2^x - 1 $.







                share|cite|improve this answer














                share|cite|improve this answer



                share|cite|improve this answer








                edited 58 mins ago

























                answered 1 hour ago









                AndreasAndreas

                7,8581037




                7,8581037























                    3












                    $begingroup$

                    The easiest way is to notice that
                    $$
                    sum_{i=0}^{x-1}2^i=sum_{i=0}^{x-1}(2^{i+1}-2^{i})=({2^1}-2^0)+(2^2-2^1)+(2^3-2^2)+dotsb+(2^x-2^{x-1})
                    $$

                    which is a telescoping series.






                    share|cite|improve this answer









                    $endgroup$


















                      3












                      $begingroup$

                      The easiest way is to notice that
                      $$
                      sum_{i=0}^{x-1}2^i=sum_{i=0}^{x-1}(2^{i+1}-2^{i})=({2^1}-2^0)+(2^2-2^1)+(2^3-2^2)+dotsb+(2^x-2^{x-1})
                      $$

                      which is a telescoping series.






                      share|cite|improve this answer









                      $endgroup$
















                        3












                        3








                        3





                        $begingroup$

                        The easiest way is to notice that
                        $$
                        sum_{i=0}^{x-1}2^i=sum_{i=0}^{x-1}(2^{i+1}-2^{i})=({2^1}-2^0)+(2^2-2^1)+(2^3-2^2)+dotsb+(2^x-2^{x-1})
                        $$

                        which is a telescoping series.






                        share|cite|improve this answer









                        $endgroup$



                        The easiest way is to notice that
                        $$
                        sum_{i=0}^{x-1}2^i=sum_{i=0}^{x-1}(2^{i+1}-2^{i})=({2^1}-2^0)+(2^2-2^1)+(2^3-2^2)+dotsb+(2^x-2^{x-1})
                        $$

                        which is a telescoping series.







                        share|cite|improve this answer












                        share|cite|improve this answer



                        share|cite|improve this answer










                        answered 1 hour ago









                        Foobaz JohnFoobaz John

                        21.7k41352




                        21.7k41352























                            0












                            $begingroup$

                            Use induction to prove it:




                            • Show it is true for $x=1$


                            • Assume it is true for $x=k$, this means that you are allowed to use $$sum_{i=0}^{k-1} 2^{i} = 2^k-1$$


                            • Then show that it is true for $n=k+1$ by using the above formula.







                            share|cite|improve this answer









                            $endgroup$


















                              0












                              $begingroup$

                              Use induction to prove it:




                              • Show it is true for $x=1$


                              • Assume it is true for $x=k$, this means that you are allowed to use $$sum_{i=0}^{k-1} 2^{i} = 2^k-1$$


                              • Then show that it is true for $n=k+1$ by using the above formula.







                              share|cite|improve this answer









                              $endgroup$
















                                0












                                0








                                0





                                $begingroup$

                                Use induction to prove it:




                                • Show it is true for $x=1$


                                • Assume it is true for $x=k$, this means that you are allowed to use $$sum_{i=0}^{k-1} 2^{i} = 2^k-1$$


                                • Then show that it is true for $n=k+1$ by using the above formula.







                                share|cite|improve this answer









                                $endgroup$



                                Use induction to prove it:




                                • Show it is true for $x=1$


                                • Assume it is true for $x=k$, this means that you are allowed to use $$sum_{i=0}^{k-1} 2^{i} = 2^k-1$$


                                • Then show that it is true for $n=k+1$ by using the above formula.








                                share|cite|improve this answer












                                share|cite|improve this answer



                                share|cite|improve this answer










                                answered 1 hour ago









                                Ben CrossleyBen Crossley

                                787318




                                787318























                                    0












                                    $begingroup$

                                    You can prove this equality with the following method:
                                    x belongs to N*
                                    For x=1, 2^0 = 2^1 - 1 = 1
                                    For x=2, 2^0 + 2^1 = 2^2 - 1 = 3
                                    .
                                    .
                                    .
                                    Assume that the equality is correct up to (x-1) and demonstrate for the next order (see attached image),



                                    Proof



                                    Thus, the equality holds for all x in N*






                                    share|cite|improve this answer








                                    New contributor




                                    Wane Mamadou is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                    Check out our Code of Conduct.






                                    $endgroup$


















                                      0












                                      $begingroup$

                                      You can prove this equality with the following method:
                                      x belongs to N*
                                      For x=1, 2^0 = 2^1 - 1 = 1
                                      For x=2, 2^0 + 2^1 = 2^2 - 1 = 3
                                      .
                                      .
                                      .
                                      Assume that the equality is correct up to (x-1) and demonstrate for the next order (see attached image),



                                      Proof



                                      Thus, the equality holds for all x in N*






                                      share|cite|improve this answer








                                      New contributor




                                      Wane Mamadou is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                      Check out our Code of Conduct.






                                      $endgroup$
















                                        0












                                        0








                                        0





                                        $begingroup$

                                        You can prove this equality with the following method:
                                        x belongs to N*
                                        For x=1, 2^0 = 2^1 - 1 = 1
                                        For x=2, 2^0 + 2^1 = 2^2 - 1 = 3
                                        .
                                        .
                                        .
                                        Assume that the equality is correct up to (x-1) and demonstrate for the next order (see attached image),



                                        Proof



                                        Thus, the equality holds for all x in N*






                                        share|cite|improve this answer








                                        New contributor




                                        Wane Mamadou is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                        Check out our Code of Conduct.






                                        $endgroup$



                                        You can prove this equality with the following method:
                                        x belongs to N*
                                        For x=1, 2^0 = 2^1 - 1 = 1
                                        For x=2, 2^0 + 2^1 = 2^2 - 1 = 3
                                        .
                                        .
                                        .
                                        Assume that the equality is correct up to (x-1) and demonstrate for the next order (see attached image),



                                        Proof



                                        Thus, the equality holds for all x in N*







                                        share|cite|improve this answer








                                        New contributor




                                        Wane Mamadou is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                        Check out our Code of Conduct.









                                        share|cite|improve this answer



                                        share|cite|improve this answer






                                        New contributor




                                        Wane Mamadou is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                        Check out our Code of Conduct.









                                        answered 1 hour ago









                                        Wane MamadouWane Mamadou

                                        1




                                        1




                                        New contributor




                                        Wane Mamadou is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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                                        New contributor





                                        Wane Mamadou is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                        Check out our Code of Conduct.






                                        Wane Mamadou is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                        Check out our Code of Conduct.























                                            0












                                            $begingroup$


                                            Applying the finite geometric series formula we obtain
                                            begin{align*}
                                            sum_{i=0}^{x-1}2^i=frac{2^x-1}{2-1}=2^x-1
                                            end{align*}







                                            share|cite|improve this answer









                                            $endgroup$


















                                              0












                                              $begingroup$


                                              Applying the finite geometric series formula we obtain
                                              begin{align*}
                                              sum_{i=0}^{x-1}2^i=frac{2^x-1}{2-1}=2^x-1
                                              end{align*}







                                              share|cite|improve this answer









                                              $endgroup$
















                                                0












                                                0








                                                0





                                                $begingroup$


                                                Applying the finite geometric series formula we obtain
                                                begin{align*}
                                                sum_{i=0}^{x-1}2^i=frac{2^x-1}{2-1}=2^x-1
                                                end{align*}







                                                share|cite|improve this answer









                                                $endgroup$




                                                Applying the finite geometric series formula we obtain
                                                begin{align*}
                                                sum_{i=0}^{x-1}2^i=frac{2^x-1}{2-1}=2^x-1
                                                end{align*}








                                                share|cite|improve this answer












                                                share|cite|improve this answer



                                                share|cite|improve this answer










                                                answered 10 mins ago









                                                Markus ScheuerMarkus Scheuer

                                                60.6k455145




                                                60.6k455145






















                                                    Jack Möller is a new contributor. Be nice, and check out our Code of Conduct.










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                                                    Jack Möller is a new contributor. Be nice, and check out our Code of Conduct.












                                                    Jack Möller is a new contributor. Be nice, and check out our Code of Conduct.
















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