Question

How do you verify the identity `1+cos10y=2cos^2 5y`?

Answer 1

See the proof below

Explanation:

We need

`cos2x=2cos^2x-1`

Let `x=5y`

Then,

`cos10y=2cos^2 (5y)-1`

So,
`1+cos10y=2cos^2 (5y)`

`LHS=RHS`