posted by shay .
Given cos a=-7/25 in quadrant II , and sin b=-12/13, in quadrant IV , find cos(a+b) and sin2a
sin a = 24/25
cos b = 5/13
now apply addition formulas
Thanks but how did you find that? My numbers came out completely different -_-
If a is in QII, sin is positive, cos is negative.
7^2 + 24^2 = 25^2, so it's a 7-24-25 triangle
If b is in QIV, sin is negative, cos is positive.
5^2 + 12^2 = 13^2, so it's a 5-12-13 triangle.
what did you do to find the missing sin/cos values?