Q$3$: Prove or disprove

$5n^2+40n+100=O(n^2).$

$\frac{n^3}{1000}=O(n^2).$

$f(n)={\displaystyle \underset{i=1}{\overset{n}{}}}i2.$ is $f(n)=\frac{O}{n^3}?$

$n^k=\frac{O}{2^n}$

${\displaystyle \underset{i=1}{\overset{n}{}}}logi=\frac{O}{nlogn}$

$7^n=(9^n)$