中大信科院 2013 复试离散数学真题第二题
为真命题，证明如下：
(1) $(\exist x)F(x)$                         P
(2) $F(c)$                                 ES,(1)
(3) $(\forall x)(F(x) \rightarrow G(x))$      P
(4) $F(c) \rightarrow G(c)$                 US,(3)
(5) $G(c)$                                 T,(2),(4),I
(6) $(\exist x)G(x)$                        EG,(5)