trig addition/subtraction formulas

[xc630]

Hi I need some help simplifying this expression. My teacher said to multiply through and then change the cot (s) to cos/sin(s). The final answer should be cos (x+y).


The expression is:

sinx siny (cotx coty -1)

I multiplied through and got this:

sinx siny cotx coty - sinx siny

Then if you convert the cot(s) you are left with:

cosx cosy - sin x siny

I don't know what to do next or if I erred somewhere before.
Thanks for any help.

 

[Unco]

What does your compound angle formula sheet say for cos(x+y)?

 

[xc630]

Oh duh.

cosx cosy - sin x siny is cos (x+y). I was thinking in terms of regular algebra and thoguh you had to get cosx + cosy first and then factor out teh cos. Thank you.