y=e^t -->y^2=e^2tx=e^-2t =y^-2 =1/y^2xy^2=1两边对x求导得y^2+2xyy'=0y'=-y/2x两边对x求导得y''=-1/2 *(y/x)' =-1/2 * (y'-1)/x^2 =（1-y')/2x^2 =(1+y/2x)/2x^2 =(2x+y)/4x^3
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