Cos (a+b) sin a=-3/5 sin b=-5/13 a & b in Q3 ?
cos(a+b)=?sin(a)=−35sin(b)=−513
cos(a+b)=cos(a)∗cos(b)−sin(a)∗sin(b)
cos(a)=? and cos(b)=?
cos(a)=√1−sin2(a)=√1−(35)2=√52−325=√165=±45=±45
cos(b)=√1−sin2(b)=√1−(513)2=√132−5213=√14413=±1213=±1213
cos(a+b)=±(45)×(1213)−(−35)×(−513)
cos(a+b)=±(45)×(1213)−(35)×(513)
cos(a+b)=(±(4∗12)5∗13)−(3∗55∗13)
cos(a+b)=±(4∗12)−3∗55∗13
cos(a+b)=±48−1565
1.) cos(a+b)=48−1565=3365=0.50769230769
2.) cos(a+b)=−48−1565=−6365=−0.96923076923
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