What is the smallest solution of the equation $x^4-34x^2+225=0$?
We would like to factor the left-hand side in the form \[
(x^2-\boxed{\phantom{09}})(x^2-\boxed{\phantom{25}}).
\] The numbers in the boxes must multiply to give $225$ and add to give $34$.  We write $225=3\cdot3\cdot5\cdot5$ and try a few different pairs until we find that 9 and 25 satisfy the requirements.  We factor further using difference of squares and solve. \begin{align*}
(x^2-9)(x^2-25)&=0 \\
(x+3)(x-3)(x-5)(x+5)&=0 \\
x = \pm 3, x=\pm 5&
\end{align*} The smallest of these solutions is $x=\boxed{-5}$.