z = ln√（x²+y²）= (1/2) ln( x²+y² )δz/δx = (1/2) * 1/( x²+y² ) * 2x = x /( x²+y² )δz/δy = (1/2) * 1/( x²+y² ) * 2y= y/( x²+y²)ln√（x²+y²）对x求导答案是(1/√(x²+y²)) ×1/2*(xˆ2+yˆ2)ˆ-1/2*(2x+2y*y＇)题目中是把y看做x的函数，从而z是x的一元函数。z = (1/2) ln( x²+y² ),表达式先化简一下，后面简单。令 u = x²+y² ,du/dx = 2x + 2y * y＇dz/dx = (1/2) * 1/u * du/dx = ( x + y * y＇) / (x²+y² )