Show that none of the following is an identity:
cos2 θ + cos θ = 1
If the given equation is an identity, then it is true for every value of θ.
So, let θ = 60°
So, for θ = 60°, consider the L.H.S. = cos2 60° + cos 60°
= (1/2)2 + (1/2)
= (1/4) + (1/2)
= 3/4 ≠ 1
Therefore, L.H.S. ≠ R.H.S.
Thus, the given equation is not an identity.