Question

已知A=x²-2x+1,B=2x²-6x+3,求3A-[（2A-B)-2(A-B)]的值，其中x= -7

已知A=x²-2x+1,B=2x²-6x+3,求3A-[（2A-B)-2(A-B)]的值，其中x= -7 怎么写？？？

Answer 1

3A-[（2A-B)-2(A-B)]
=3A-(2A-B-2A+2B]
=3A-B
=3(x²-2x+1)-(2x²-6x+3)
=3x²-6x+3-2x²+6x-3
=x²
=(-7)²
=49
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Answer 2

3A-[(2A-B)-2(A-B)]

=3A-(2A-B-2A+2B)
=3A-2A+B+2A-2B
=3A-B
=3(x^2-2x+1)-(2x^2-6x+3)
=3x^2-6x+3-2x^2+6x-3
=x^2
当x=-7时，原式=(-7)^2=49

Answer 3

3A-[(2A-B)-2(A-B)]
=3A-(2A-B-2A+2B)
=3A-B (1)
将A=x^2-2x+1,B=2x^2-6x+3带入(1)式
3(x^2-2x+1)-(2x^2-6x+3)=x^2=(-7)^2=49