方法一：代换法设长方体的长宽高分别是x,y,z则表面积f(x,y,z)=2(xy+yz+xz)xyz=a³∴z=a³/xyf(x,y)=2(xy+a³/x+a³/y)令∂f/∂x=2x-2a³/x²=0∂f/∂y=2y-2a³/y&#...