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