Question

丨x1-x2丨如何变成（x1+x2）²-4x1x2

Answer 1

是不是“丨x1-x2丨²如何变成（x1+x2）²-4x1x2”或者：“丨x1-x2丨如何变成√[x1+x2）²-4x1x2]
因为：丨x1-x2丨是变不成（x1+x2）²-4x1x2的
1、丨x1-x2丨²
=x1²-2x1x2+x2²
=x1²+2x1x2+x2²-4x1x2
=（x1+x2）²-4x1x2

2、丨x1-x2丨
=√(x1-x2)²
=√[(x1²-2x1x2+x2²)]
=√[(x1²+2x1x2+x2²-4x1x2)]
=√[(x1+x2）²-4x1x2]

Answer 2

(x1-x2)²
=x1²-2x1x2+x2²
=x1²+2x1x2+x2²-4x1x2
=(x1+x2)²-4x1x2

追问

是不是错了？没给根号就加平方。

追答

没错
采纳吧

Answer 3

丨x1-x2丨=√(x1-x2)²=√(x1²-2x1x2+x2²)
=√(x1²+2x1x2+x2²-4x1x2)
=√（x1+x2）²-4x1x2

追问

题目是，已知二次函数y=x²+kx+6的图像与X轴的正半轴交于AB两点 且AB两点间的距离为2，求k的值.要推导出AB=根号b²-4ac/a的绝对值

Answer 4

（丨x1-x2丨）²=x1²+x2²-2*(x1*x2)
=x1²+x2²+2*(x1*x2)-4*(x1*x2)
=（x1+x2）²-4x1x2