## Elementary Technical Mathematics

$$\angle 1= \angle3=90^o.$$ $$\angle 2= \angle4=90^o.$$
Since $\angle 1, \angle 3$ are suplementray, then $\angle 1+ \angle 3=180$. They are equal since they are vertical angles, so $\angle 1= \angle 3$, and hence $$2\angle 1=180$$ that is $$\angle 1= \angle3=90^o.$$ Similarly, $\angle 2, \angle 4$ are vertical angles and $\angle 3+ \angle 4=360-180=180$, and hence $$2\angle 3=180$$ that is $$\angle 3= \angle4=90^o.$$