Question

solve on copy plz prove fast

Answer 1

we know that sum of two angles of quadrilateral is 180.
$\cos( \frac{1}{2}(180 ) ) + \cos( \frac{1}{2}(180) ) \\ = \cos(90) + \cos(90) \\ = 0 + 0 \\ = 0$
we know that cos 90°= 0