Prove that Sin^2x - cos^2x - 1 = cos x

Answer 1

fundamental trig identity, which provides the basis for many others:

sin^2 + cos^2 = 1

so,

sin^2 - cos^2 - 1
= 1 - cos^2 - cos^2 - 1
= -2cos^2
??

as written, not true. Is there a typo somewhere?