# If N is a Positive Integer, Prove that 3 3 N − 26 N − 1 is Divisible by 676. - Mathematics

If n is a positive integer, prove that $3^{3n} - 26n - 1$  is divisible by 676.

#### Solution

$3^{3n} - 26n - 1 = {27}^n - 26n - 1 . . . \left( 1 \right)$

$\text{ Now, we have: }$

${27}^n = (1 + 26 )^n$

$\text{ On expanding, we get }$

$(1 + 26 )^n = ^{n}{}{C}_0 \times {26}^0 +^{n}{}{C}_1 \times {26}^1 + ^{n}{}{C}_2 \times {26}^2 + ^{n}{}{C}_3 \times {26}^3 +^{n}{}{C}_4 \times {26}^4 + . . . ^{n}{}{C}_n \times {26}^n$

$\Rightarrow {27}^n = 1 + 26n + {26}^2 [^{n}{}{C}_2 + ^{n}{}{C}_3 \times {26}^1 + ^{n}{}{C}_4 \times {26}^2 + . . . ^{n}{}{C}_n \times {26}^{n - 2} ]$

$\Rightarrow {27}^n - 26n - 1 = 676 \times \text{ an integer }$

${27}^n - 26n - 1 \text{ is divisible by } 676$

$\text{ Or, }$

$3^{3n} - 26n - 1 \text{ is divisible by } 676 \left( \text{ From } (1) \right)$

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#### APPEARS IN

RD Sharma Class 11 Mathematics Textbook
Chapter 18 Binomial Theorem
Exercise 18.1 | Q 8 | Page 12