设函数z=ln(1+x2+y2),求dz|1,2
设函数z=ln(1+x2+y2),求dz|1,2
【正确答案】:dz=(∂z/∂x)dx+(∂z/∂y)dy =[2x(1+x2+y2)]dx+[2y/(1+x2+y2)]dy, 故dz|(1,2)=[(2×1)/(1+12+22)]dx+[(2×2)/(1+12+22)]dy =(1/3)dx+(2/3)dy.
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