# Math Help - calculating cos(A+B)

1. ## calculating cos(A+B)

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

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

Am I wrong or the back? Please explain

2. Originally Posted by skeske1234
If cos A = 1/3 with 0 < A< pi/2, and sinB=1/4, with pi/2<B<pi. calculate cos(A+B).

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

Am I wrong or the back? Please explain
you are correct