Prove Trig Identity

[gkagawa]

Prove cosx = cos2x

I tried using the double angle identity for the RHS to get this:

= cos^2x - sin^2x
= (cosx - sinx) (cosx + sinx)

What next?

 

[galactus]

\(\displaystyle cos(x)=cos(2x)\) is not an identity. Are you solving for x?.

 

[Deleted member 4993]

Prove cosx = cos2x

I tried using the double angle identity for the RHS to get this:

= cos^2x - sin^2x
= (cosx - sinx) (cosx + sinx)

What next?

In general "cos(x) = cos(2x)" is not an identity. This equation is true only for certain values of "x".

So you cannot "prove" the statement.....