Question:
In Fig. 16.20, find the measure of ∠MPN..png)
Solution:
Since the sum of all the angles of a quadrilateral is $360^{\circ}$, we have:
$45^{\circ}+90^{\circ}+90^{\circ}+\angle \mathrm{MPN}=360^{\circ}$
$\Rightarrow 225^{\circ}+\angle \mathrm{MPN}=360^{\circ}$
$\therefore \angle \mathrm{MPN}=135^{\circ}$