隱函數微分答案疑問?

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圖片參考：http://imgcld.yimg.com/8/n/AD01636710/o/101112231200613869609771.jpg

希望對你有幫助...

___________________ ___x³-xy+y³=1___3x²-[x(dy/dx)+y]+3y²(dy/dx)=0_____3x²-x(dy/dx)-y+3y²(dy/dx)=0________________(3y²-x)(dy/dx)=y-3x²_________________________dy/dx=(y-3x²)/(3y²-x)When x=1, y=1, dy/dx=(1-3)/(3-1)_________________________dy/dx=-1From (3y²-x)(dy/dx)=y-3x²______________(3y²-x)(d²y/dx²)=dy/dx-3(2x)______+(dy/dx)[3(2y)(dy/dx)-1]______________(3y²-x)(d²y/dx²)=dy/dx-6x_________+(dy/dx)[6y(dy/dx)-1]When x=1, y=1__(3-1)(d²y/dx²)+(-1)[6(-1)-1]=-1-6__________________2(d²y/dx²)+7=-7____________________2(d²y/dx²)=-14_______________________d²y/dx²=-7∴正解對

正解好像是-7，是哪一個對呢?
正解對