x1三次方+8X2+20
=x1³+3x1²+x1+8x2+20-3x1²-x1
=x1(x1²+3x1+1)+8x2+20-3x1²-x1 ………………x1²+3x1+1=0
=8x2+20-3x1²-x1
=-3x1²-9x1+8x1+8x2+20 …………………………3x1²+9x1=-3
=-3+8(x1+x2)+20 …………………………X1+X2=-3
=-3+8*(-3)+20
=-7
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