Question:
Make the correct alternative in the following question:
For all $n \in \mathbf{N}, 3 \times 5^{2 n+1}+2^{3 n+1}$ is divisible by
(a) 19
(b) 17
(c) 23
(d) 25
Solution:
Let $\mathrm{P}(n)=3 \times 5^{2 n+1}+2^{3 n+1}$, for all $n \in \mathbf{N}$.
For $n=1$,
$\mathrm{P}(1)=3 \times 5^{2+1}+2^{3+1}$
$=3 \times 5^{3}+2^{4}$
$=375+16$
$=391$
$=17 \times 23$
For $n=2$,
$\mathrm{P}(2)=3 \times 5^{4+1}+2^{6+1}$
$=3 \times 5^{5}+2^{7}$
$=9375+128$
$=9503$
$=17 \times 13 \times 43$
As, $\operatorname{HCF}(391,9503)=17$
So, $\mathrm{P}(n)$ is divisible by 17 .
Hence, the correct alternative is option (b).