If ∑n=1∞an=S\sum_{n=1}^{\infty} a_n = S∑n=1∞an=S, which of the following is necessarily true?
∑n=1∞an+1=S−a1\sum_{n=1}^{\infty} a_{n+1} = S - a_1∑n=1∞an+1=S−a1
∑n=1∞2an=S\sum_{n=1}^{\infty} 2a_n = S∑n=1∞2an=S
ana_nan is monotonic
limn→∞an=S\lim_{n \to \infty} a_n = Slimn→∞an=S