if a and b are the measures of two first quadrant angles and sin a = 4/5 and sin b = 5/13, find sin a+b

Question

if a and b  are the measures of two first quadrant angles and sin a = 4/5 and sin b = 5/13, find sin a+b

dumbcow · Answer

angle addition property
$$\sin(a+b) = \sin a \cos b + \sin b \cos a$$

dumbcow · Answer

also
$$\sin^{2} \theta + \cos^{2} \theta = 1$$
this is how you can find cos(a) and cos(b)

anonymous · Answer

I tried to to that but got (20/39)+(1/3) or 11/13. The available answers are 
(63/65) 
(33/65)
(16/65) 
(56/65)

anonymous · Answer

@dumbcow

dumbcow · Answer

looks like you didn't get the right values for cos(a) and cos(b)

cos a = 3/5

cos b = 12/13

anonymous · Answer

So it would be sin(a+b)=(4/5)(12/13)+(3/5)(5/13) = 63/65?

dumbcow · Answer

correct

anonymous · Answer

Thanks!

dumbcow · Answer

try to go back and see what you did wrong in finding the cosine values