3x²=x+1
所以6x³=2x×3x²
=2x(x+1)
=2x²+2x
所以原式=2x²+2x+7x²-5x+1990
=9x²-3x+1990
=3(3x²-x)+1990
=3×1+1990
=1993
3x^2-x=1
3x^2-x-1=0
6x^2-2x-2=0
6x^3+7x^2-5x+1990
=x(6x^2-2x-2)+9x^2-3x+1990
=9x^2-3x-3+1993
=3(3x^2-x-1)+1993
=0+1993
=1993
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