Prove that every geometric sequence allows for $S_n(S_{3n}-S_{2n})=(S_{2n}-S_{n})^2$












3















$S_n(S_{3n}-S_{2n})=(S_{2n}-S_{n})^2$




Is there a way to prove this without expanding everything based on the geometric sum formula? I get lost very easily when trying to solve this conventionally and I feel that I am missing an obvious solution.










share|cite|improve this question



























    3















    $S_n(S_{3n}-S_{2n})=(S_{2n}-S_{n})^2$




    Is there a way to prove this without expanding everything based on the geometric sum formula? I get lost very easily when trying to solve this conventionally and I feel that I am missing an obvious solution.










    share|cite|improve this question

























      3












      3








      3








      $S_n(S_{3n}-S_{2n})=(S_{2n}-S_{n})^2$




      Is there a way to prove this without expanding everything based on the geometric sum formula? I get lost very easily when trying to solve this conventionally and I feel that I am missing an obvious solution.










      share|cite|improve this question














      $S_n(S_{3n}-S_{2n})=(S_{2n}-S_{n})^2$




      Is there a way to prove this without expanding everything based on the geometric sum formula? I get lost very easily when trying to solve this conventionally and I feel that I am missing an obvious solution.







      sequences-and-series geometric-series






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked 2 days ago









      daedsidog

      22916




      22916






















          2 Answers
          2






          active

          oldest

          votes


















          5














          Write



          $$S_n = a sum_{k=0}^{n-1} r^k$$



          Then



          $$S_{2 n} = S_n + r^n S_n$$
          $$S_{3 n} = S_{2 n} + r^{2 n}S_n$$



          The above result follows from simple algebra.






          share|cite|improve this answer





















          • This is perfect. Thank you.
            – daedsidog
            2 days ago



















          3














          firsly:
          $$S_n=a_1frac{1-r^n}{1-r}$$
          so we can say that:
          $$S_nleft(S_{3n}-S_{2n}right)=a_1^2frac{1-r^n}{1-r}left(frac{(1-r^{3n})-(1-r^{2n})}{1-r}right)=a_1^2frac{(1-r^n)(r^{2n}-r^{3n})}{(1-r)^2}$$
          and:
          $$(S_{2n}-S_n)^2=left[a_1frac{1-r^{2n}}{(1-r)}-a_1frac{1-r^n}{(1-r)}right]^2=left[a_1frac{r^n-r^{2n}}{(1-r)}right]^2=a_1^2frac{(r^n-r^{2n})^2}{(1-r)^2}$$
          now we just need to show that the top of the fractions are equivalent:
          $$(r^n-r^{2n})^2=(r^n)^2-2r^nr^{2n}+(r^{2n})^2=r^{2n}-2r^{3n}+r^{4n}$$
          $$(1-r^n)(r^{2n}-r^{3n})=r^{2n}-r^{3n}-r^{3n}+r^{4n}=r^{2n}-2r^{3n}+r^{4n}$$
          so the two are equivalent






          share|cite|improve this answer





















            Your Answer





            StackExchange.ifUsing("editor", function () {
            return StackExchange.using("mathjaxEditing", function () {
            StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
            StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
            });
            });
            }, "mathjax-editing");

            StackExchange.ready(function() {
            var channelOptions = {
            tags: "".split(" "),
            id: "69"
            };
            initTagRenderer("".split(" "), "".split(" "), channelOptions);

            StackExchange.using("externalEditor", function() {
            // Have to fire editor after snippets, if snippets enabled
            if (StackExchange.settings.snippets.snippetsEnabled) {
            StackExchange.using("snippets", function() {
            createEditor();
            });
            }
            else {
            createEditor();
            }
            });

            function createEditor() {
            StackExchange.prepareEditor({
            heartbeatType: 'answer',
            autoActivateHeartbeat: false,
            convertImagesToLinks: true,
            noModals: true,
            showLowRepImageUploadWarning: true,
            reputationToPostImages: 10,
            bindNavPrevention: true,
            postfix: "",
            imageUploader: {
            brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
            contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
            allowUrls: true
            },
            noCode: true, onDemand: true,
            discardSelector: ".discard-answer"
            ,immediatelyShowMarkdownHelp:true
            });


            }
            });














            draft saved

            draft discarded


















            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3060904%2fprove-that-every-geometric-sequence-allows-for-s-ns-3n-s-2n-s-2n-s-n%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown

























            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            5














            Write



            $$S_n = a sum_{k=0}^{n-1} r^k$$



            Then



            $$S_{2 n} = S_n + r^n S_n$$
            $$S_{3 n} = S_{2 n} + r^{2 n}S_n$$



            The above result follows from simple algebra.






            share|cite|improve this answer





















            • This is perfect. Thank you.
              – daedsidog
              2 days ago
















            5














            Write



            $$S_n = a sum_{k=0}^{n-1} r^k$$



            Then



            $$S_{2 n} = S_n + r^n S_n$$
            $$S_{3 n} = S_{2 n} + r^{2 n}S_n$$



            The above result follows from simple algebra.






            share|cite|improve this answer





















            • This is perfect. Thank you.
              – daedsidog
              2 days ago














            5












            5








            5






            Write



            $$S_n = a sum_{k=0}^{n-1} r^k$$



            Then



            $$S_{2 n} = S_n + r^n S_n$$
            $$S_{3 n} = S_{2 n} + r^{2 n}S_n$$



            The above result follows from simple algebra.






            share|cite|improve this answer












            Write



            $$S_n = a sum_{k=0}^{n-1} r^k$$



            Then



            $$S_{2 n} = S_n + r^n S_n$$
            $$S_{3 n} = S_{2 n} + r^{2 n}S_n$$



            The above result follows from simple algebra.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered 2 days ago









            Ron Gordon

            122k14154263




            122k14154263












            • This is perfect. Thank you.
              – daedsidog
              2 days ago


















            • This is perfect. Thank you.
              – daedsidog
              2 days ago
















            This is perfect. Thank you.
            – daedsidog
            2 days ago




            This is perfect. Thank you.
            – daedsidog
            2 days ago











            3














            firsly:
            $$S_n=a_1frac{1-r^n}{1-r}$$
            so we can say that:
            $$S_nleft(S_{3n}-S_{2n}right)=a_1^2frac{1-r^n}{1-r}left(frac{(1-r^{3n})-(1-r^{2n})}{1-r}right)=a_1^2frac{(1-r^n)(r^{2n}-r^{3n})}{(1-r)^2}$$
            and:
            $$(S_{2n}-S_n)^2=left[a_1frac{1-r^{2n}}{(1-r)}-a_1frac{1-r^n}{(1-r)}right]^2=left[a_1frac{r^n-r^{2n}}{(1-r)}right]^2=a_1^2frac{(r^n-r^{2n})^2}{(1-r)^2}$$
            now we just need to show that the top of the fractions are equivalent:
            $$(r^n-r^{2n})^2=(r^n)^2-2r^nr^{2n}+(r^{2n})^2=r^{2n}-2r^{3n}+r^{4n}$$
            $$(1-r^n)(r^{2n}-r^{3n})=r^{2n}-r^{3n}-r^{3n}+r^{4n}=r^{2n}-2r^{3n}+r^{4n}$$
            so the two are equivalent






            share|cite|improve this answer


























              3














              firsly:
              $$S_n=a_1frac{1-r^n}{1-r}$$
              so we can say that:
              $$S_nleft(S_{3n}-S_{2n}right)=a_1^2frac{1-r^n}{1-r}left(frac{(1-r^{3n})-(1-r^{2n})}{1-r}right)=a_1^2frac{(1-r^n)(r^{2n}-r^{3n})}{(1-r)^2}$$
              and:
              $$(S_{2n}-S_n)^2=left[a_1frac{1-r^{2n}}{(1-r)}-a_1frac{1-r^n}{(1-r)}right]^2=left[a_1frac{r^n-r^{2n}}{(1-r)}right]^2=a_1^2frac{(r^n-r^{2n})^2}{(1-r)^2}$$
              now we just need to show that the top of the fractions are equivalent:
              $$(r^n-r^{2n})^2=(r^n)^2-2r^nr^{2n}+(r^{2n})^2=r^{2n}-2r^{3n}+r^{4n}$$
              $$(1-r^n)(r^{2n}-r^{3n})=r^{2n}-r^{3n}-r^{3n}+r^{4n}=r^{2n}-2r^{3n}+r^{4n}$$
              so the two are equivalent






              share|cite|improve this answer
























                3












                3








                3






                firsly:
                $$S_n=a_1frac{1-r^n}{1-r}$$
                so we can say that:
                $$S_nleft(S_{3n}-S_{2n}right)=a_1^2frac{1-r^n}{1-r}left(frac{(1-r^{3n})-(1-r^{2n})}{1-r}right)=a_1^2frac{(1-r^n)(r^{2n}-r^{3n})}{(1-r)^2}$$
                and:
                $$(S_{2n}-S_n)^2=left[a_1frac{1-r^{2n}}{(1-r)}-a_1frac{1-r^n}{(1-r)}right]^2=left[a_1frac{r^n-r^{2n}}{(1-r)}right]^2=a_1^2frac{(r^n-r^{2n})^2}{(1-r)^2}$$
                now we just need to show that the top of the fractions are equivalent:
                $$(r^n-r^{2n})^2=(r^n)^2-2r^nr^{2n}+(r^{2n})^2=r^{2n}-2r^{3n}+r^{4n}$$
                $$(1-r^n)(r^{2n}-r^{3n})=r^{2n}-r^{3n}-r^{3n}+r^{4n}=r^{2n}-2r^{3n}+r^{4n}$$
                so the two are equivalent






                share|cite|improve this answer












                firsly:
                $$S_n=a_1frac{1-r^n}{1-r}$$
                so we can say that:
                $$S_nleft(S_{3n}-S_{2n}right)=a_1^2frac{1-r^n}{1-r}left(frac{(1-r^{3n})-(1-r^{2n})}{1-r}right)=a_1^2frac{(1-r^n)(r^{2n}-r^{3n})}{(1-r)^2}$$
                and:
                $$(S_{2n}-S_n)^2=left[a_1frac{1-r^{2n}}{(1-r)}-a_1frac{1-r^n}{(1-r)}right]^2=left[a_1frac{r^n-r^{2n}}{(1-r)}right]^2=a_1^2frac{(r^n-r^{2n})^2}{(1-r)^2}$$
                now we just need to show that the top of the fractions are equivalent:
                $$(r^n-r^{2n})^2=(r^n)^2-2r^nr^{2n}+(r^{2n})^2=r^{2n}-2r^{3n}+r^{4n}$$
                $$(1-r^n)(r^{2n}-r^{3n})=r^{2n}-r^{3n}-r^{3n}+r^{4n}=r^{2n}-2r^{3n}+r^{4n}$$
                so the two are equivalent







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered 2 days ago









                Henry Lee

                1,773218




                1,773218






























                    draft saved

                    draft discarded




















































                    Thanks for contributing an answer to Mathematics Stack Exchange!


                    • Please be sure to answer the question. Provide details and share your research!

                    But avoid



                    • Asking for help, clarification, or responding to other answers.

                    • Making statements based on opinion; back them up with references or personal experience.


                    Use MathJax to format equations. MathJax reference.


                    To learn more, see our tips on writing great answers.





                    Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


                    Please pay close attention to the following guidance:


                    • Please be sure to answer the question. Provide details and share your research!

                    But avoid



                    • Asking for help, clarification, or responding to other answers.

                    • Making statements based on opinion; back them up with references or personal experience.


                    To learn more, see our tips on writing great answers.




                    draft saved


                    draft discarded














                    StackExchange.ready(
                    function () {
                    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3060904%2fprove-that-every-geometric-sequence-allows-for-s-ns-3n-s-2n-s-2n-s-n%23new-answer', 'question_page');
                    }
                    );

                    Post as a guest















                    Required, but never shown





















































                    Required, but never shown














                    Required, but never shown












                    Required, but never shown







                    Required, but never shown

































                    Required, but never shown














                    Required, but never shown












                    Required, but never shown







                    Required, but never shown







                    Popular posts from this blog

                    1300-talet

                    1300-talet

                    Display a custom attribute below product name in the front-end Magento 1.9.3.8