Question

If cos2A=11/25, show that cosA=6/5√2​

Answer 1

it is not I think...

Explanation:

Using the identity

`\cos(2a) = 2 \cos^2(a) - 1`

We can solve:

`2 \cos^2(a) - 1 = (11)/(25)`

`2 \cos^2(a) = (36)/(25)`

`\cos^2(a) = (18)/(25)`

`\cos(a) = \sqrt (18)/(25)`

`\cos(a) = \sqrt{ 2 \cdot (3^2)/(5^2) }= \frac35 \sqrt 2`

So whoever asked the question forgot the factor 2!!!

Or did I miss anything??