题目

要10道化简再求值,10道合并同类项 急明天就要交了!

提问时间：2021-03-01

答案

（1）（x+2y)(x-2y)-(2x-y)(-2-y),其中x=8,y=-8
(2) (1-2a)(1+2a)(1+4a平方）（1+16a四次方）,其中a=-二分之一
(3)（a^2 ）b-(ab)^2+3a^2 b,其中a=1/2,b=1/3
(4) 9x+6x^2-3(x-2/3x^2),其中x=-3
(5) 3x^2-xy/9x^2-6xy+y^2,其中X=1/2 Y=-2/3
(6) 3x^2y-2xy^2-xy^2+2xy^2其中x=-1,y=2 先化简,再求值
(7) 3x-2y-5xy-（2x-3y+2xy）,其中x+y=4,xy=-7
(8) 化简求值3x²y-[2xy²-2（xy-3/2x²y)+xy]+3xy²其中x=3、y=-1/3
1.（x+2y)(x-2y)-(2x-y)(-2-y)
=x^2-4y^2-(-4x-2xy+2y+y^2)
=x^2-4y^2+4x+2xy-2y-y^2
=x^2-5y^2+4x+2xy-2y
当x=8,y=8时
x^2-5y^2+4x+2xy-2y=64-5*64+32+2*64-16
=64*（1-5+2）+32-16
=-128+16=-112
2.(1-2a)(1+2a)(1+4a^2)(1+16a^4)
=(1-4a^2)(1+4a^2)(1+16a^4)
=(1-16a^4)(1+16a^4)
=1-256a^8
当a=1/2时
1-256a^8=1-256*(1/2)^8=1-1=0
(3) 原式=2(a^2)b-ab^2=ab(2a-b)=(1/6)(1-1/3)=1/9
(4) 9X+ 6X^2 - 3(X - 2/3 X^2)
= 9X + 6X^2 -3X + 2X^2
= 6x + 8x^2
因为 X=-3
则：原式＝-18 +72
= 54
(5) (3x²-xy)/(9x²-6xy+y²)
=x(3x-y)/(3x-y)²
=x/(3x-y)
=(1/2)/(3/2+2/3)
=1/2*6/13
=3/13
(6)
=3x^2y-2xy^2-xy^2+2xy^2
=3x^2y-xy^2
=3*(-1)^2*2-(-1)*2^2
=6+4
=10
(7) 3x-2y-5xy-（2x-3y+2xy）
=3x-2y-5xy-2x+3y-2xy
=x+y-7xy
=4-7×(-7)
=4+49
=53
(8) 3/2x²y=(3/2)x²y
3x²y-[2xy²-2（xy-(3/2)x²y)+xy]+3xy²
=3x²y-[2xy²-2xy+3x²y+xy]+3xy²
=3x²y-2xy²+2xy-3x²y-xy+3xy²
=xy²+xy
=xy(y+1)
=3*(-1/3)*(-1/3+1)
=-2/3