[chocoholic4lyf]

Let x and y be real numbers such that:
sin(x) + sin(y)= a
cos(x)+cos(y)= b

determine sin(x+y) and cos(x+y).
any help would be appreciated
thanks in advance

[cth]

You need to use the equations (see http://en.wikipedia.org/wiki/Cosinus):
cos²x + sin²x = 1
cos(x+y) = cos(x)cos(y) - sin(x)sin(y)
sin(x+y) = sin(x)cos(y) + cos(x)sin(y)

With the equations you already have:

sin(x) + sin(y)= a
cos(x)+cos(y)= b

and a bit of work up, you should be able to solve your problem.

btw, I didn't notice there was a math forum as well. Nice 

[Ranadeep]

Some more formula  you might need : )

2Sinx Cosx = Sin(2x)
Cos²X-Sin²X = Cos(2x)
Also take a note on Trigonometric transformations : )