We will learn how to express the multiple angle of cos 2A in terms of tan A.
Trigonometric function of cos 2A in terms of tan A is also known as one of the double angle formula.
We know if A is a number or angle then we have,
cos 2A = cos^2 A - sin^2 A
cos 2A = cos^2 A - sin^2A/cos^2 A ∙ cos^2 A
⇒ cos 2A = cos^2 A (1 - tan^2 A)
⇒ cos 2A = 1/sec^2 A (1 - tan^2 A)
⇒ cos 2A = 1 - tan^2 A/1 + tan^2 A