(Siny - cosy) (siny + cosy)

Question

I need to simplify using Pythagorean identities I need hel asap

Answer 1

This is a difference in perfect squares, if we expand

 

sin²y - cos²y =

 

-(cos²y - sin²y) =

 

-cos(2y)

Answer 2

Note you have a difference of squares in its factored form.

 

(siny - cosy) (siny + cosy) = sin²y - cos²y = -(cos²y - sin²y) = -cos(2y)