Question

An arithmetic series has a common difference of 7. Given that the sum of the first 20 terms of the series is 530, how do you find the first term of the series?

Answer 1

`-5`

Explanation:

With the first term as a and common difference d, the sum for n terms = `na+(n(n-1))/2d`, using 1 + 2 + 3 +..(n-1) = (n-1)(n)/2
Here, n = 20 and d = 7.

20a + ((19)(20)/2)7 = 530

`20a = -100`. So, a = `-5`.