MCQ
For all $n \in N, 3.5^{2n+1}+ 2^{3n+1}$ is divisible by:
- A$19$
- ✓$17$
- C$23$
- D$25$
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$I$. If $9$ divides $a^3+b^3+c^3$, then $3$ divides $a b c$.
$II$. If $9$ divides $a^3+b^3+c^3+d^3$, then $3$ divides abcd. Then
$\frac{3}{1^2} + \frac{5}{{{1^2} + {2^2}}} + \frac{7}{{{1^2} + {2^2} + {3^2}}} + ...... + $ up to $20$ terms is equal to $\frac{k}{{21}}$, then $k$ is equal to