a_n:=0 \forall n \in N \\
b_n:=|x`_{p_n}-c|\\
c_n:=|x``_{p_n}-x`_{p_n}|+|x`_{p_n}-c|\\
~ t.d. ~ a_n \leq b_n \leq c_n~ ~\forall n \in N \Longrightarrow 0 = \lim_{n \in N} a_n \leq \lim_{n \in N} b_n \leq \lim_{n \in N} c_n = 0