If cos A = 1/3 with 0 < A< pi/2, and sinB=1/4, with pi/2<B<pi. calculate cos(A+B).

Question

My answer is (-sqrt15-2sqrt2)/12
But the back of the book says the answer is (-sqrt15+2sqrt2)/12

Am I wrong or the back? Please explain

Answer 1

You are right, I got the same answer.
And then I checked our answer by finding the actual angles.
Unless you typed the quadrant incorrectly, the book is wrong