Given sin(a) = 4/5 and cos(b) = − 1/6 , with a and b both in the interval [𝜋/2 , 𝜋] , find the exact values of sin(a + b) and cos(a − b).

Answer 1

In QII

sin a = 4/5 ⇒ cos a = -3/4 Learn to use Pythagoras

cos b = -1/6 ⇒ sin b = √35 / 6

Now substitute values into appropriate formula.