Question

Find the value of `cos2theta` for `theta=pi/4`?

Answer 1

`cos2theta=0`

Explanation:

`cos2theta=cos(theta+theta)`

= `costhetaxxcostheta-sinthetaxxsintheta`

as for `theta=pi/4`, `costheta=sintheta=1/sqrt2`

`cos2theta=costhetaxxcostheta-sinthetaxxsintheta`

= `1/sqrt2xx1/sqrt2-1/sqrt2xx1/sqrt2`

= `1/2-1/2=0`