
Sin Cos Tan...
I thought I knew how to work out these equations, but my answer was different from the answer they gave.
Given: a and B are firstquadrant angles, sin a = 5/13 and cos B = 3/5, evaluate cos (a+B).
I used the equations sin^2 a + cos^2 a = 1 to get cos a = 12/13. That would make cos (a+B) 99/65. However, their answer was 16/65. Help, please? Another one:
Given: a and B are firstquadrant angles, sin a = 5/13 and cos B = 3/5, evaluate sin (aB).
I used the same equations to get sin B = 4/5, and thus sin (aB) = 27/65. Their answer was 33/65. :confused: Please help me out!!!
~Cassie

You got wrong answers because you thought
cos(a+B) = cos(a) +cosB, and sin(aB) = sin(a) sinB.
No.
The correct expansions are:
cos(a+B) = cos(a)*cosB sin(a)*sinB.
And, sin(aB) = sin(a)*cosB cos(a)*sinB
So, get all the sines and cosines of angles "a" and B, then solve your questions again. You will get them this time.

Ah, I reworked them and got it! Thanks so much!!!!