因式分解,,想知部找

(1) rs + 2r + s + 2
= r(s + 2) + (s + 2)
= (r + 1)(s + 2)
(2) -9f^2 + 3f + 2
= -9f^2 - 3f + 6f + 2
= -3f(3f + 1) + 2(3f + 1)
= (3f + 1)(2 - 3f)
(3) 4m^2 + mn - 3n^2
= 4m^2 - 3mn + 4mn - 3n^2
= m(4m - 3n) + n(4m - 3n)
= (m + n)(4m - 3n)
(4) x^2/4 + 2x + 1請複查問題有沒有打錯.