[tex]\frac{d}{dx}[f(2x)] = x^2[/tex]

I let u(x) = 2x, then

[tex]\frac{d}{dx}[f(u)] = \frac{dy}{du} \frac{du}{dx}[/tex]

[tex]\frac{d}{dx}[f(2x)] = 2 \frac{dy}{du}[/tex]

therefore

[tex]\frac{dy}{du} = \frac{1}{2}x^2[/tex]

then

[tex]f'(x) = \frac{dy}{dx} = \frac{dy}{du} \frac{du}{dx}[/tex]

[tex] = (\frac{1}{2}x^2) (2)[/tex]

[tex] = x^2[/tex]

why doesn't this work??