Question

If cos cos A=3/5 and sinB=7/25,where A is acute and B is obtuse,find without using tables the value of Cos(A+B)

Answer 1

but cos(A+B) = cos A cos B - sin A sin B
for angle A 3^2+4^2 = 5^2
cos A = 3/5 so sin A = 4/5
for angle B 24^2 + 7^2 = 25^2
cos B = -24/25 and sin B = 7/25 because B is obtuse

Answer 2

Not clear