Question

giúp em với ạ 

Answer 1

Đặt x=a/2

=>tan x=cot x-2cot 2x

=>\(tanx-cotx=-2\cdot\dfrac{cos2x}{sin2x}\)

=>\(\dfrac{sinx}{cosx}-\dfrac{cosx}{sinx}=\dfrac{-2\cdot cos2x}{sin2x}\)

=>\(\dfrac{sin^2x-cos^2x}{sinx\cdot cosx}=\dfrac{-cos2x}{sinx\cdot cosx}\)

=>sin^2x-cos^2x=-cos2x(luôn đúng)