若x平方减5x加2等于0,则x的4次方减3x的立方减7x的平方减x加7等于
x^2-5x+2=0
x^2=5x-2
因为x^4-3x^3-7x^2-x+7
=(x^4-3x^3-7x^2)-x+7
=(x^2)(x^2-3x-7)-x+7
=(5x-2)(5x-2-3x-7)-x+7
=(5x-2)(2x-9)-x+7
=10x^2-4x-45x-x+7+18
=10x^2-50x+7+18
=10(5x-2)-50x+25
=50x-20-50x+25
=25-20
=5
整体代入法
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