Changing the bound of summation

Question

Would someone be able to clarify and explain how the following statement equal this:

$\sum_{i=1}^{n+1} Xi$=$\sum_{i=1}^n Xi+X_{n+1}$

Answer 1

$\sum_{i=1}^{n+1}X_i=X_1+\cdots+X_n+X_{n+1}=(X_1+\cdots+X_n)+X_{n+1}=\sum_{i=1}^{n}X_i+X_{n+1}$