b/a-b+1/a (读作：a分之b减b加1/a) 如果是“b/a减a分之b加1”那就是b/a-(b+1)/a
=(b/a+1/a)-b 即：b/a-(b+1)/a
=(b+1)/a-b =【b-(b+1)】/a
=(b+1)/a-ab/a =【b-b-1】/a
=(b+1-ab)/a (读作：a分之b加1减ab) = -1/a