Find the time that a ball needs to dropm from 80m if $g=10frac{m}{s^2}$

We know that $S=frac{1}{2}*g*t^2=>t=sqrt(frac{2*S}{g})$ we put the values and find the time needen.

If given $f(x)=frac{1}{x}$ find the $limf(x)$ as x aproaches infinty

$limf(x)=0$

Given $f(x)=2x^2+5cosx$ find the $frac{df}{dx}$

$frac{df}{dx}=4x-5sinx$