求下列隐函数的微分或导数: (Ⅰ)设ysinx-cos(x-y)=0,求dy; (Ⅱ)设方程确定y=y(x),求y’与y’’.

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问题 求下列隐函数的微分或导数:
(Ⅰ)设ysinx-cos(x-y)=0,求dy;
(Ⅱ)设方程确定y=y(x),求y’与y’’.

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答案(Ⅰ)利用一阶微分形式不变性求得 d(ysinx)-dcos(x-y)=0, 即sinxdy+ycosxdx+sin(x-y)(dx-dy)=0, 整理得[sin(x-y)-sinx]dy=[ycosx+sin(x-y)]dx, 故 [*] (Ⅱ)将原方程两边取对数,得等价方程 [*]ln(x2+y2)=arctan[*](*) 现将方程两边求微分得 [*] 化简得xdx+ydy=xdy-ydx,即(x-y)dy=(x+y)dx, 由此解得 [*] 为求y’’,将y’满足的方程(x-y’)y’=x+y两边再对x求导,即得 (1-y’)y’+(x-y)y’’=1+y’[*] 代入y’表达式即得 [*]

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