Question 1169171
<pre>
Is this what you mean?

{{{tan(4x) = (4tan(x) - 3tan^2(x))/(1 - 6tan^2(x) + tan^4(x))}}}

If so, then this is not an identity.  For let x = 45°

{{{tan(4*45^o) = (4tan(45^o) - 3tan^2(45^o))/(1 - 6tan^2(45^o) + tan^4(45^o))}}}

{{{tan(180^o) = (4(1) - 3(1)^2)/(1 - 6(1) + (1)^4)}}}

{{{0 = (4 - 3)/(1 - 6 + 1)}}}

{{{0 = 1/(-4)}}}

{{{0=-1/4}}}

That's false, so the equation is not an identity so we can't prove
what's false is true!

Edwin</pre>