4)(x+1)/(x+1)平方 + 1/(x-1)
=(x-4)/(x+1)(x-1) 乘以 (x+1)平方/(x-4)(x+1) + 1/(x-1)
=(x-4)/(x+1)(x-1) 乘以 (x+1)/(x-4) + 1/(x-1)
=1/(x-1)+1/(x-1)
=2/(x-1)