December 9th, 2015, 10:56 AM  #1 
Newbie Joined: Dec 2015 From: Indonesia Posts: 1 Thanks: 0  Please help my homework
I really don't know how to answer this. Please help me. The question is : If a and b are 90 or below degrees, and sin a  sin b equals 3/5 and cos a + cos b equals 4/5. How much is sin (a + b). Thank you 
December 9th, 2015, 02:36 PM  #2 
Global Moderator Joined: Dec 2006 Posts: 19,291 Thanks: 1683 
There's something weird about this question, as specifying "a and b are 90 or below degrees" doesn't help in obtaining the answer. If it were also stated that a and b are positive, there would be no such a and b that satisfy the equations. My guess is that the condition was a failed attempt to ensure that the answer is positive. If you square and add the given equations, you get 2cos(a)cos(b)  2sin(a)sin(b) + 2 = 1, so cos(a + b) = 1/2. It follows that sin(a + b) = ±$\sqrt3/2$. Either sign is possible. 

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