Question

(sinx−cosx)(sin x+cos x)=1 − 2cos²x

I need help with the steps there should be 4

Answer 1

use foil/difference of squares to expand (sinx−cosx)(sin x+cos x)

then use the identity sin^2(x) + cos^2(x) = 1 to re-write your problem in terms of cos only. (major hint: solve the identity for cos^2(x))