Question

Which of these are NOT a trigonometric identity?

A.) 1-cos^2x=sin^2x
B.) sin^2x+1=cos^2x
C.) 1+cot^2x=csc^2x
D.) sec^2x-1=tan^2x

Answer 1

NOT to be a trig identity requires a single exception

so take an angle for which you know the trig rations and test the equations
e.g. let x = 45°
you know sin^2 45 = cos^2 45 = 1/2
tan^2 45 = 1 and cot^2 45= 1
also sec^2 45 = 2, and csc^2 45 = 2

What do you think?