(sinB+cosB)(sinB-cosB)=2sin²B-1

(sin b + cos b)(sin b – cos b) = 2 sin² b – 1
sin² b – cos² b = 2 sin² b – 1
sin² b – (1 – sin² b) = 2 sin² b – 1
sin² b – 1 + sin² b = 2 sin² b – 1
2 sin² b – 1 = 2 sin² b – 1  ⇒  Terbukti