Originally Posted by

**looi76** For the following function f(x), find f'(x) and any intervals in which f(x) is increasing:

$\displaystyle x^4 + 4x^3$

$\displaystyle f(x) = x^4 + 4x^3$

$\displaystyle f'(x) = 4x^3 + 12x^2$

$\displaystyle f'(x) = 0$

$\displaystyle 4x^3+12x^2 = 0$

$\displaystyle 4x^2(x+3)$

$\displaystyle 4x^2$

$\displaystyle x^2 = \frac{1}{4}$

$\displaystyle x = \sqrt{\frac{1}{4}}$

$\displaystyle x = 0.5$ The answer is wrong. where did I go wrong?

$\displaystyle x+3$

$\displaystyle x = -3$