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Prove-that-3-3n-26n-1-is-divisible-by-676-where-n-Z-




Question Number 2545 by Rasheed Soomro last updated on 22/Nov/15
Prove that 3^(3n) −26n−1 is divisible by 676,  where n∈Z^+
Provethat33n26n1isdivisibleby676,wherenZ+
Commented by Yozzi last updated on 22/Nov/15
Use induction.
Useinduction.
Answered by Yozzi last updated on 22/Nov/15
Let P(n)=3^(3n) −26n−1=(27)^n −26n−1  P(1)=27−26−1=0=676×0  P(2)=27^2 −52−1=676=1×676  P(3)=27^3 −26×3−1=29×676    Let P(n) be the proposition that, ∀n∈Z^+ ,                      P(n)=676m  where  P(n)=27^n −26n−1 and m∈Z^≥ .  For n=1, P(1)=27−26−1=0=676×0  0∈Z^≥ ⇒m=0 ∴ P(n) is true for n=1.    Assume P(n) is true for n=k, i.e                        P(k)=676m  m∈Z^≥ .  For n=k+1, observe the difference  Δ=P(k+1)−P(k).   P(k+1)=27^(k+1) −26(k+1)−1  Δ=27×27^k −26k−26−1−27^k +26k+1  Δ=26×27^k −26  Δ=26(27^k −1)  ⇒P(k+1)=P(k)+26(27^k −1)  P(k+1)=676m+26(27^k −1)  Now, 27^k −1=(27−1)(Σ_(r=0) ^(k−1) 27^r )=26(Σ_(r=0) ^(k−1) 27^r )  ∴P(k+1)=676m+676Σ_(r=0) ^(k−1) 27^r   P(k+1)=676(m+Σ_(r=0) ^(k−1) 27^r )=676q  where q∈Z^+  since {m+Σ_(r=0) ^(k−1) 27^r }∈Z^+ .  ⇒ P(k+1) is true if P(k) is true.  Since P(1) is true then, by P.M.I,  P(n) is true ∀n∈Z^+ .
LetP(n)=33n26n1=(27)n26n1P(1)=27261=0=676×0P(2)=272521=676=1×676P(3)=27326×31=29×676LetP(n)bethepropositionthat,nZ+,P(n)=676mwhereP(n)=27n26n1andmZ.Forn=1,P(1)=27261=0=676×00Zm=0P(n)istrueforn=1.AssumeP(n)istrueforn=k,i.eP(k)=676mmZ.Forn=k+1,observethedifferenceΔ=P(k+1)P(k).P(k+1)=27k+126(k+1)1Δ=27×27k26k26127k+26k+1Δ=26×27k26Δ=26(27k1)P(k+1)=P(k)+26(27k1)P(k+1)=676m+26(27k1)Now,27k1=(271)(k1r=027r)=26(k1r=027r)P(k+1)=676m+676k1r=027rP(k+1)=676(m+k1r=027r)=676qwhereqZ+since{m+k1r=027r}Z+.P(k+1)istrueifP(k)istrue.SinceP(1)istruethen,byP.M.I,P(n)istruenZ+.
Commented by Rasheed Soomro last updated on 22/Nov/15
NICE^(Very)  !
NICEVery!
Answered by Rasheed Soomro last updated on 22/Nov/15
AN   EASY Approach  p(n):   3^(3n) −26n−1 is divisible by 676  Case−1_(−)  :For n=1   3^(3n) −26n−1=3^3 −26−1=0 and     676∣0  Hence p(n) is true for n=1  Case−2 : Assume that for n=k , p(n) is true  I−e   676 ∣ (3^(3k) −26k−1)  Now for n=k+1, we have           3^(3(k+1)) −26(k+1)−1         =3^(3k+3) −26k−26−1         =27.3^(3k) −26k−27         =27.3^(3k) −27(26k)−27+26(26k)         =27(3^(3k) −26k−1)+676k       ∵  676 ∣ (3^(3k) −26k−1) [By hypothesis]]              and 676 ∣ 676k        ∴ 676 ∣ 27(3^(3k) −26k−1)+676k  ∴  p(n) is true for n=k+1,if it is true for n=k                 QED
ANEASYApproachp(n):33n26n1isdivisibleby676Case1:Forn=133n26n1=33261=0and6760Hencep(n)istrueforn=1Case2:Assumethatforn=k,p(n)istrueIe676(33k26k1)Nowforn=k+1,wehave33(k+1)26(k+1)1=33k+326k261=27.33k26k27=27.33k27(26k)27+26(26k)=27(33k26k1)+676k676(33k26k1)[Byhypothesis]]and676676k67627(33k26k1)+676kp(n)istrueforn=k+1,ifitistrueforn=kQED
Commented by Yozzi last updated on 22/Nov/15
Nicely  □
Nicely◻
Commented by Rasheed Soomro last updated on 23/Nov/15
□ THANKS!  Actually I saw this problem and its solution in a book  and wanted to share with friends on this platform.  Therefore in order to get oppertunity I posted the question.       I like your answer because its your own whereas my  answer is bookish and not my own.Anyway it is such that  it be shared.
◻THANKS!ActuallyIsawthisproblemanditssolutioninabookandwantedtosharewithfriendsonthisplatform.ThereforeinordertogetoppertunityIpostedthequestion.Ilikeyouranswerbecauseitsyourownwhereasmyanswerisbookishandnotmyown.Anywayitissuchthatitbeshared.

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