Question

Let → a = ⟨ 1 , − 3 ⟩ and → b = ⟨ − 3 , k ⟩. Find k so that →a and → b will be orthogonal (form a 90 degree angle).

Answer 1

if a b is orthogonal , a•b will be 0
a•b=1x-3+-3k=0
so k=-1