Question

Find the sum of the sequence 46+47+48+49+...+137

Answer 1

8418

Explanation:

1 + 2 + ... + n is (n^2 + n)/2

46 + 47 + ... + 137

is the same as

1 + 2 + ... + 137 - (1 + 2 + ... + 45)

or

(137^2 + 137)/2 - (45^2 + 45)/2

= 8418