What is the sum of the arithmetic series 6 + 12 + 18 + ... + 60?

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Answer 1 · 2016-01-22T01:09:47

The common difference $d = 12 - 6 = 6$ $d=12-6=6$

Now, determine what term is 60?

$a_{n} = a_{1} + (n - 1) d$ $a_n=a_1+(n-1)d$

$60 = 6 + (n - 1) (6)$ $60=6+(n-1)(6)$

$n = 10$ $n=10$

Now that we know 60 is the 10th term , use the sum formula ...

sum $= \frac{a_{1} + a_{n}}{2} \times n$ $=(a_1+a_n)/2xxn$

sum $= \frac{6 + 60}{2} \times 10 = 330$ $=(6+60)/2xx10=330$

hope that helped