Question

if cosx = 3/5 , find exact value of cos2x ?

Answer 1

\[\cos(2x) = \cos^2 (x) - \sin^2(x)\]

Answer 2

I hope you also know that \(\sin^2(x) = 1 - \cos^2(x)\)

Answer 3

So we want\[2\ cos^2(x) + 1\]

Answer 4

Also, \(\cos^2 \theta = \cos \theta \cdot \cos \theta\).
And we know that \(\cos \theta= \dfrac{3}{5}\).

So you should know what to do next.