因为1-√t=-(1+√t)+2
(1-√t)/(1+√t)=((2-1)-√t)/(1+√t)=(2-1-√t)/(1+√t)=(2-(1+√t))/(1+√t)=2/(1+√t) - (1+√t)/(1+√t)=2/(1+√t) -1= -1 + 2/(1+√t)
