答案
(a-1)/a过程a^2 - 2a +1 = (a - 1)^2 = (a - 1) ( a - 1 )a - a^2 = a ( 1 - a )右侧相比( a - 1 ) / ( - a ) = ( 1 - a ) / a之前那个答案好像出了点问题，抱歉