Find the values of sinx and tanx if cosx=4/5 and x is in the first quadrant.
Alternatively,
$\displaystyle Sin^2x+Cos^2x=1\Rightarrow\ Sin^2x=1-Cos^2x=1-\frac{16}{25}=\frac{25}{25}-\frac{16}{25}=\frac{9}{25}$
$\displaystyle \Rightarrow\ Sinx=\frac{3}{5}$
as Sin(x) is positive in the first quadrant.
$\displaystyle Tanx=\frac{Sinx}{Cosx}=\frac{3\left(\frac{1}{5} \right)}{4\left(\frac{1}{5} \right)}=\frac{3}{4}$