Question

甚麽是一元二次方程ax^2+bx+c (a≠0 且△=b^2-4ac≥0)中 设两个根为X1和X2 则X1+X2= -b

Answer 1

不妨设x1≤x2

x1=(-b-√△)/2a

x2=(-b+√△)/2a

＝＞x1+x2=(-b-√△)/2a+(-b+√△)/2a=-b/a

x1×x2=(-b-√△)/2a×(-b+√△)/2a=c/a