Let XXX have the cumulative distribution function F(x)=x24F(x) = \frac{x^2}{4}F(x)=4x2 for 0≤x≤20 \le x \le 20≤x≤2. What is the PDF f(x)f(x)f(x)?
x4\frac{x}{4}4x
x2\frac{x}{2}2x
x312\frac{x^3}{12}12x3
x22\frac{x^2}{2}2x2