# Math Help - proving an identity

1. ## proving an identity

prove that

2sin2a - 3cos2a - 3sina + 3 = sina(4cosa + 6sina - 3)

i am very tired and cant seem to figure it out. would be grateful for any help.

2. Hello,

sin(2a)=2cos(a)sin(a)
cos(2a)=cos²(a)-sin²(a)=(1-sin²(a))-sin²(a)=1-2sin²(a)

If you replace these values in the initial expression, you'll get what you want ;-)