let Phi_n denote the sentence for all x_1 for all x_2 ...for all x_{n-1} there exists x_n (neg (x_{n} =x_1) ) /\ neg (x_{n} =x_2) /\.../\

neg(x_{n} = x_{n-1} ).

how can I show that every model of Phi_n must have at least n elements in the underlying set .