1 + 2 + 3 + ... + 98 + 99 + 100

101 + 102 + 103 + ... + 998 + 999 + 1000

Step-by-step explanation:

Explanation:

This series is arithmetic since the common difference is 1. In other words, we're adding 1 to each term to get the next one.

The quickest way to find the sum is to use the formula below

S = (n/2)*(a+b)

where n is the number of terms, 'a' is the first term, and b is the last (or nth) term. This formula only works for arithmetic series.

We can see that n = 100, a = 1 and b = 100, so

S = (n/2)*(a+b)

S = (100/2)*(1+100)

S = 50*101

S = 5050

This can be confirmed by noticing that we have 100/2 = 50 pairs of terms. Pair up the terms such that 1 goes with 100, 2 goes with 99, 3 goes with 98, and so on.

We have these fifty sums

1+100 = 101

2+99 = 101

3+98 = 101

48+53 = 101

49+52 = 101

50+51 = 101

We have 50 copies of 101 being added, which leads to 50*101 = 5050

The slowest and most tedious way to confirm this is to type 1+2+3+...+98+99+100 into your calculator. The result should say 5050. Using a spreadsheet would be a better option instead of using a handheld calculator. I don't recommend this unless you need more proof that we end up with 5050 as the final answer.