Trigonometry
posted by Cyndy .
Find the counterexample to shows that the equation sec (alpha)  cos (alpha) = sin (alpha) sec (alpha) is not an identity.
Please explain.

there are many counterexamples. For instance,
sec(pi/4)  cos(pi/4) = √2  1/√2
sin(pi/4)*sec(pi/4) = 1/√2 * √2 = 1
not equal. In fact, for any alpha not a multiple of pi, they are not equal.