(1)求A-[3B-2(A-B)]
=A-[3B-2A+2B]
=A-3B+2A-2B
=3A-5B
∵A=3x-5x^2-7 , B=x^3+x-3x^2
∴原式=3(3x-5x^2-7 )-5(x^3+x-3x^2 )
=9x-15x²-21-5x³-5x+15x²
=-5x³+4x-21
(2)若x=-1.5,求(1)中代数式的值
原式=-5*(-3/2)³+4*(-3/2)-21
=-5*(-27/8)-6-21
=27*5/8 -27
=27*3/8
=81/8
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