Question

If ∆ABC is right-angled at C, then cos (A + B) = ?
(a) 0
(b) $\frac{1}{2}$
(c) 1
(d) $\frac{\sqrt{3}}{2}$

Answer 1

(a) 0

∠A + ∠B + ∠C = 180^o (by angle sum property)
It is given that ∠C = 90^o.
∴ ∠A + ∠B = 90^o
cos (A + B) = cos (90^o) = 0