trig
posted by Michael on .
Find cos(a=b). sin a = 3/5, a lies in Quadrant II, and cos b = 5/13, b lies in Quadrant I.

since the = and + sign are on the same key, I will assume cos(a=b) is a typo and you meant
cos(a+b)
cos(a+b) = cos(a)cos(b)  sin(a)sin(b)
since a is in II and sin(a) = 3/5, cos(a) = 4/5
since b in in I and cos(b) = 5/13, sin(b) = 12/13
so cos(a+b) = (4/5)(5/13)  (3/5)(12/13)
= (2036)/65
= 56/65