Đáp án:

\(f(x) = x f'(x) - 2x^3 - 3x^2 \quad \) Chia hai vế cho \(x^2\)

\(\Rightarrow \frac{f'(x)}{x} = \frac{f(x)}{x^2} + 2x + 3 \Leftrightarrow \frac{x f'(x) - f(x)}{x^2} = 2x + 3\)

\(\Leftrightarrow \left(\frac{f(x)}{x}\right)' = 2x + 3  \Leftrightarrow \frac{f(x)}{x} = x^2 + 3x + C\)

\(f(1) = 4 \Rightarrow C = 0\)

\(\Rightarrow  f(x) = x^3 + 3x^2 \Rightarrow  f(2) = 20 \quad \Rightarrow \boxed{\text{B}}\)

Gợi ý: \( \left(\frac{f(x)}{x}\right)' = \frac{x f'(x) - f(x)}{x^2}\)