Given that tan (a) = 7/15 with (a) in the first quadrant and cos (b) = 7/20 with (b)in the fourth quadrant. Find cos (a - b)?

Answer 1

if tan a = 7/15, the sin a = 7/sqrt(274) and cos a = 15/sqrt(274)

If cos b (4th quadrant) = 7/20, then sin b = -sqrt(351)/20

Now use those values and the addition formula to get your answer

cos(a-b)=cos a cos b + sin a sin b and watch your signs!!!!!!!!!!!!!!!