Solve the problem using a python code $12\mathrm{.}15$ Perform the same computation as in Sec. $12\mathrm{.}2,$ but for the truss depicted in Fig. P$12\mathrm{.}15.$

FIGURE P$12\mathrm{.}15$

$12\mathrm{.}2$ If the input to reactor $3$ in Sec. $12\mathrm{.}1$ is decreased $25$ percent, use the matrix inverse to compute the percent change in the concentration of reactors $2$ and $4.$