X1和X2是一元二次方程2X^2+5X-2=0的两根求下列各值

X1和X2是一元二次方程2X^2+5X-2=0的两根求下列各值
x1+x2=-2.5,x1x2=-1
1.|X1-X2|
=根号（6.25+4）
=0.5根号41
2.1/X1^2+1/X2^2
=（6.25+2）/1
=8.25
3.X1^3+X2^3
=(x1+x2)(x1^2-x1x2+x2^2)
=(-2.5)*(6.25+3)
=-23.125那些数是怎么拆出来的1.|X1-X2|=根号[（x1+x2)^2-4x1x2]....2.1/X1^2+1/X2^2=[（x1+x2)^2-2x1x2]/(x1x2)^2.....3.X1^3+X2^3=(x1+x2)(x1^2-x1x2+x2^2)=(x1+x2)[(x1+x2)^2-3x1x2]....谢谢啊