Answer to Question #226121 in Calculus for Ram

Question

Prove that 2 to the power n+1 > (n + 2) · sin(n) for all positive integers n. 

Answer 1

Consider the function $f(x)=2^{x+1}-(x+2).$

$f'(x)=2^{x+1}\ln2-1\geq2^{1+1}\ln2-1=\ln(\dfrac{4}{e})>0, x\geq1$

Then the function $f(x)$ increases for $x\geq1.$

$f(1)=2^{1+1}-(1+2)=1>0$

Hence

$2^{n+1}>(n+2), n\geq 1$ $1\geq\sin n, n>1$

Then

$2^{n+1}>n+2\geq(n+2)\cdot\sin n, n\geq1$

Therefore $2^{n+1}>(n+2)\cdot\sin n,$ for all positive integers $n.$