使用mathcode求简支梁的内力

"简支梁集中力计算
'<hr /><div style = "max-width:180mm;">
'<img style="width:190pt;" alt = "简支梁受力.png" class="side" src = "http://app.52midas.com/images/%E7%AE%80%E6%94%AF%E6%A2%81%E5%8F%97%E5%8A%9B.png">
'<p><b>参数</b></p>
'梁长 -'l = 5'm
'荷载-'F = 10'kN
'距离-'a = 2'm
#post
'<p><b>内力</b></p>
'弯矩
M_max = F*(l - a)*a/l'kN·m
'剪力
V_A = F*(l - a)/l'kN
V_B = -F*a/l'kN
'<p><b>内力图</b></p>
#hide
PlotWidth = 600
PlotHeight = 150
#show
'求'x_1 = 1'm时的内力
#post
'弯矩图
M(x) = V_A*x - F*(x - a)*(x > a)
$Plot{-M(x) @ x = 0 : l}
M(x_1)'kN·m
'剪力图
V(x) = V_A - F*(x > a)
$Plot{V(x) @ x = 0 : l}
V(x_1)'kN