# Math Help - Find sin (x + y)

1. ## Find sin (x + y)

Angle x is acute, angle y is obtuse, cos x = 15/17
and sin y = 3/5. Find sin (x + y).

After doing the math, I found sin (x + y) to be 77/85.

The textbook tells me that the answer is 13/85.

I know 13/85 would be the answer if we were looking for
sin (x MINUS y), right?

2. sin(x+y) = sin(x)cos(y) +sin(y)cos(x)

Since angle y is a second quadrant angle cos(y)< 0 so you use cos(y) = -4/5 instead of 4/5

sin(x+y) = (-32+45)/85

3. ## ok...

Originally Posted by Calculus26
sin(x+y) = sin(x)cos(y) +sin(y)cos(x)

Since angle y is a second quadrant angle cos(y)< 0 so you use cos(y) = -4/5 instead of 4/5

sin(x+y) = (-32+45)/85
I now see what I did wrong. Thanks.