The Formula for the Sum of the First N Terms $\frac { n \left( a _ { 1 } - a _ { n } \right) } { 2 }$

Multiple Choice

The formula for the sum of the first n terms of an arithmetic sequence is _______.

A) $\frac { n \left( a _ { 1 } - a _ { n } \right) } { 2 }$
B) $\frac { n \left( a _ { 1 } + a _ { n - 1 } \right) } { 2 }$
C) $a _ { 1 } + ( n - 1 ) d$
D) $\frac { n \left( a _ { 1 } + a _ { n } \right) } { 2 }$