楼上做的莫非是（x+y)×(x-y)?
（x+y)/(x-y)
= (X - Y + 2Y)/(X - Y)
= 1 + 2Y/(X - Y)
要使此式最大,则Y尽可能大,X - Y为正数且尽可能小.
因此X = 50,Y = 49
此时原式 = 1 + 2×49/1 = 99