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x+y=1①
x²+y²=2②
①²-②,得2xy=-1,xy=-1/2
x⁷+y⁷=(x³+y³)(x⁴+y⁴)-x⁴y³-x³y⁴
=(x³+y³)[(x²+y²)²-2x²y²]-x³y³(x+y)
=(x+y)(x²+y²-xy)[(x²+y²)²-2x²y²]-x³y³(x+y)
=1*(2-xy)(4-2x²y²)-x³y³*1
=(2+1/2)(4-1/2)+1/8
=71/8
=
x²+y²=2②
①²-②,得2xy=-1,xy=-1/2
x⁷+y⁷=(x³+y³)(x⁴+y⁴)-x⁴y³-x³y⁴
=(x³+y³)[(x²+y²)²-2x²y²]-x³y³(x+y)
=(x+y)(x²+y²-xy)[(x²+y²)²-2x²y²]-x³y³(x+y)
=1*(2-xy)(4-2x²y²)-x³y³*1
=(2+1/2)(4-1/2)+1/8
=71/8
=
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把y=–x+1带入x^2+y^2=2,解出x,y的值,再求x^7+y^7=?
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x+y=1
x²+y²=2
xy=[(x+y)²-(x²+y²)]/2=(1²-2)/2=-1/2
x³+y³=(x+y)(x²-xy+y²)=1×(2+1/2)=5/2
x^4+y^4=(x²+y²)²-2(xy)²=2²-2×(-1/2)²=4-1/2=7/2
x^7+y^7=(x^4+y^4)(x³+y³)-x³y³(x+y)=(7/2)×(5/2)-(-1/2)³×1=35/4+1/8=71/8
x²+y²=2
xy=[(x+y)²-(x²+y²)]/2=(1²-2)/2=-1/2
x³+y³=(x+y)(x²-xy+y²)=1×(2+1/2)=5/2
x^4+y^4=(x²+y²)²-2(xy)²=2²-2×(-1/2)²=4-1/2=7/2
x^7+y^7=(x^4+y^4)(x³+y³)-x³y³(x+y)=(7/2)×(5/2)-(-1/2)³×1=35/4+1/8=71/8
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