How to solve this pattern sum of 11 12 13 14 15 16 17 18 19 20?

Answer 1

A linear series with "i" terms can be summed by finding the average of the first and last terms, then multiplying by the amount of terms.This sequence can be written as follows:

ai= 11 + 1(i-1)

= 11 + i - 1

= 10 + i

11 is the first term, 1 is the difference between terms which gets multiplied by (i-1)

So the fourth term a4= 10 + 4 = 14

Sum total = i * (a1+ ai)/2 (no. of terms * average of first and last term)

= 10 * (11 + 20)/2

= 10 * (31/2)

= 155