1.x-s=±（x-t)
x=(s+t)/2
2.x-5√ x+6=0
(√ x-2)(√ x-3)=0
x=4 x=9
3.x/（x-1)=(k-2x)x/(x-1)
x^2=k-2x
x^2+2x-k=0
△=0
k=-1