Assertion(A): sec 2θ-tan 2θ=1 for all values of θ≠90°.
Reason(R): sin 2θ+cos 2θ=1
(1)
Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion. If you divide the basic Pythagorean identity
sin 2θ+cos 2θ=1 by cos 2θ then you get 1+tan 2θ=sec 2θ⇒sec 2θ-tan 2θ=1.