Sin^2x-cos^2x

Answer 1

is a trig ratio piece
= 1

Answer 2

sin^2 x + cos^2 x = 1
but
since cos (2x) = cos^2 x - sin^2 x

sin^2 x-cos^2 x = - sin(2x)

Answer 3

argggh
meant to say:
sin^2 x - cos^2 x = - cos(2x)