# cos 2A in Terms of tan A

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 = cos2 A  -  sin2 A

cos 2A = $$\frac{cos^{2} A - sin^{2} A}{cos^{2} A}$$ ∙ cos2 A

⇒ cos 2A = cos2 A (1 - tan2 A)

⇒ cos 2A =  $$\frac{1}{sec^{2} A}$$(1 - tan2 A)

⇒ cos 2A = $$\frac{1 - tan^{2} A}{1 + tan^{2} A}$$