Posted by MIK on Thursday, May 17, 2012 at 4:43pm.
sin a = 15/17 , with a in I, so cos a = 8/17
cos b = 4/5, with b in IV, then sin b = -3/5
cos(a+b) = cosa cosb - sina sinb
= (8/17)(4/5) - (15/17)(-3/5)
= (32 + 45)/85
= 77/85
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