1.δz/δx=x^2 e^(x+y)+2x e^(x+y)
2.δz/δy=x^2 e^(x+y)
3.dz=(δz/δx)dx+(δz/δy)dy=x^2 e^(x+y)+2x e^(x+y)dx+x^2 e^(x+y)dy
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