问两道初一的数学题

来源：百度知道 编辑：UC知道 时间：2024/05/26 03:46:16
1.若x^2-px+ab=(x+a)(x+b),则p=.....................( )
A.-a+b B.-a-b c.a-b D.a+b
2.计算（要过程）4（x+2y）^2-25(x-y)^2

(x+a)(x+b)
=x^2+ax+bx+ab
=x^2+(a+b)x+ab
=x^2-px+ab
对应项系数相等
所以-p=a+b
p=-a-b
选B

4（x+2y）^2-25(x-y)^2
=4(x^2+4xy+4y^2)-25(x^2-2xy+y^2)
=4x^2+16xy+16y^2-25x^2+50xy-25y^2
=-21x^2+66xy-9y^2

1.B
x^2-px+ab=(x+a)(x+b)=x^2+(a+b)x+ab
系数项相等
-p=a+b
p=-a-b

2.
4（x+2y）^2-25(x-y)^2
=[2(x+2y)]^2-[5(x-y)]^2
=[2(x+2y)+5(x-y)][2(x+2y)-5(x-y)]
=(2x+4y+5x-5y)(2x+4y-5x+5y)
=(7x-y)(-3x+9y)
=-3(7x-y)(x-3y)

1: B
因为(x+a)(x+b)=x^2+(a+b)x+ab
所以 -p=(a+b) p=-a-b
2: 4(x+2y)^2-25(x-y)^2
=4x^2+16xy+16y^2-25x^2-50xy-25y^2
=-21x^2-34xy-9y^2
=-(3x+y)(7x+9y)

1.B
2.4(X^2+4XY+4Y^2)-25(X^2-2XY+Y^2)
=4X^2+16XY+4Y^2-25X^2+50XY-25Y^2
=-21X^2+66XY-21Y^2

b.
[(2x+4y)+5(x-y)][(2x+4y)-5(x-y)]=-(7x-y)(3x-9y)