Question

What is the sum of an 11–term arithmetic sequence where the first term is 5 and the last term is 55?

300

330

360

390

Answer 1

The amswer is 330. Each number in the sequence increases by 5.

Answer 2

Gauss' forrmula sum the first and last term of the series and divide by the number of pairs n/2

Answer 3

\[s _{n}=\frac{n}{2}\left( a _{1}-a _{n} \right)\]\[s _{11}=\frac{11}{2}(5+55)=330\]

Answer 4

@kiss.. think of it this way: suppose they were asking you to sum 11,000 terms, not 11 terms! the Gauss formula is easy to remember and useful