# Math Help - Rolle's Theorem

1. ## Rolle's Theorem

Prove that the equation x^7+x^5+x^3+1=0 as exactly one real solution. You should use Rolle's Theorem at some point in the proof.

Any help? Thanks!

2. Originally Posted by friday616
Prove that the equation x^7+x^5+x^3+1=0 as exactly one real solution. You should use Rolle's Theorem at some point in the proof.

Any help? Thanks!
If we let $f(x) = x^7+x^5+x^3+1$, since $f(-2) < 0$ and $f(0) > 0,$ and $f$ is continuous, then by the intermediate value theorem, there's at least on root on $[-2,0]$. To prove there's only one root, suppose there are two, say $f(a) = f(b) = 0$. By Rolles theorem, there is a $c$ in $(a,b)$ where $f'(c) = 0.$ Further, $f'(x)$ must change sign crossing $x = c$ (i.e. goes up levels out and comes down or vice-versa). Now $f'(x) = 7x^6+5x^4+3x^2 \ge 0$ (it never changes sign). Thus, we have a contradiction - there can't be two roots.