Given: … eq. 1

And we know that, …eq. 2

We need to find the value of cos A.

…eq. 3

(∵, sin² θ +cos² θ =1

⇒ cos² A = 1-sin² A

⇒ cos A = √ (1-sin² A)

Substituting eq. 1 in eq. 3, we get

cos A = √(1-1/4)

=

Substituting values of sin A and cos A in eq. 2, we get