For a natural number \(n\), we call the number \(1+2+3+\cdots + n\) the \(n^{\text{th}}\) triangular number, and we denote it by \(T_n\). Find \(T_n+T_{n-1}\) in terms of \(n\).
For a natural number \(n\), define the \(n^{\text{th}}\) triangular number by \(T_n=1+2+3+\cdots+n\).
Show that \(3T_n+T_{n-1}\) is itself a triangular number, and determine which one.