Question

Find the sum of the first 20 terms of the sequence {4,10,16,22,...} step by step

Answer 1

All terms separated by 6 so #20 = 4 + 6 * 19 = 118

(if the first term were 6 instead of 4 the 20th would be 120)

Also, #10 = 58 and #11 = 64 (you can use the same reasoning)

The 1st and 20th term add to 4 + 118 = 122

The 2nd and 19th add to 10 + 112 = 122

Likewise 10th and 11th add to 58 + 64 = 122

So 10 * 122 = 1220