求解对流微分方程：u_t+a(x,t)u_x=0，具体说明在程序中，四种经典格式，写成类的形式，VC6.0环境 -solving differential equations convection : a u_t (x, t) = 0 u_x specify the procedures, Four classic format, type written in the form of environmental VC6.0