13. $a, b$ are the roots of the equation $x^{2}-4 x+$ $1=0$, $c, d$ are the roots of the equation $x^{2}$ $-5 x+2=0$, and let
$$
\begin{aligned}
t= & \frac{a}{b+c+d}+\frac{b}{a+c+d} \\
& +\frac{c}{a+b+d}+\frac{d}{a+b+c} .
\end{aligned}
$$

Then express $\frac{a^{2}}{b+c+d}+\frac{b^{2}}{a+c+d}+\frac{c^{2}}{a+b+d}+\frac{d^{2}}{a+b+c}$ in terms of $t$.
13. $9 t-9$