# Help finding inflection points & concavity

I'm trying to find the inflection points and concavity of the graph of

$$x^4 - 50x^2 + 2$$

The first derivative is

$$4x^3 - 100x$$

and the second derivative is

$$12x^2 - 100$$

Trying to solve for x I keep getting

$$\sqrt{(25/3)}$$

and the online assignment thing keeps telling me I'm getting it wrong. am I missing something?

Thank you!

• Note that $\sqrt {\frac{25}3}=\frac 53\sqrt3$ – Mark Bennet Apr 12 '14 at 17:46
• The second derivative is a quadratic, so it should have 2 roots. Your answer is correct but forgot the negative of that square root. – Dylan Apr 12 '14 at 19:44

You are nearly correct, just forgot to include $\pm$. There are two roots, opposite in sign.