Posted by **Michael** on Sunday, August 30, 2009 at 10:03pm.

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

- trig -
**Reiny**, Monday, August 31, 2009 at 12:27am
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)

= (-20-36)/65

= -56/65

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