Posted by jj reddick on Sunday, October 24, 2010 at 11:23pm.
suppose sin(x)=4/5 and cos(y)=12/13, and x and y are in the second quadrant. determine the value of each expression: sin(x+y), cos(xy), cos2x, sin2y.

trig  Reiny, Sunday, October 24, 2010 at 11:31pm
you should recognize the 3,4,5 and the 5,12, 13 rightangled triangles, so
if sinx = 4/5, then cosx = 3/5 in II
if cosy = 1213, then siny = 5/13 in II
to have this type of question, you must also have come across the expansions for sin(A+B), cos(AB), etc.
I will do cos(xy)
cos(xy) = cosx cosy + sinx siny
= (3/5)(12/13) + (4/5)(5/13)
= 56/65
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