Question

Which of the following are trigonometric identities ? Check all that apply. Tan^2 x -

Answer 1

I hope you are looking for these answer choices as you have not posted the choices. Therefore, I have a similar question with these answer choices.

a). tan² x - 1 = sec²x
b). 4 cosxsinx = 2cos + 1 -2sinx
c). tanx cosx cscx = 1
d). 1- tanx tany = cos(x+y) ÷ cosx cosy

Options B, C, D are correct because, for all them, left-hand side equals to the right-hand side for x = 0.

The option A)
tan² x - 1 = sec²x is not true. 
since x = 0
Therefore,
The Right hand side would be:
                                         tan² x - 1 = sec²(0)
                                                             = 1
The Left hand side would be:
                                                    tan² x - 1 = tan² (0) - 1
                                                                   = 0 - 1
                                                                   = -2
Therefore,
Both sides are not equal so it's not a trigonometric identity.