Answer
$ m \angle A \approx 83.3^{o}$
Work Step by Step
Apply law of cosines $a^2=b^2+c^2-2bc \cos A$
Need to solve for $A$
$2bc \cos A=b^2+c^2-a^2$
This implies $\cos A=\cos^{-1}(\dfrac{b^2+c^2-a^2}{2bc})$
Plug the given values to obtain:
$A=\cos^{-1}(\dfrac{(14)^2+(16)^2-(20)^2}{2 \times 14 \times 16}) =\cos^{-1}(\dfrac{52}{448})$
In order to calculate the value, we will use calculator in degree mode.
$ m \angle A \approx 83.3^{o}$
