Sum of Integers from 1 to n

In this lesson, we'll learn how to find the sum of numbers from 1 to n.

What does the sum of integers from 1 to nn mean?

Natural numbers are positive numbers starting from 11. These can be written as:

1,2,3,4,5,6,7,8,9,10......1,2,3,4,5,6,7,8,9,10......

We want to write a program that takes a number and sums up all the numbers from 11 up to that number.

For example, if n=5n = 5, then the sum of numbers from 11 to 55 is: 1+2+3+4+5=151 + 2 + 3 + 4 + 5 = 15.

Mathematical Notation

The sum of all numbers up to a number is equal to the sum of that number and the sum of all the numbers before it. Let’s write it down mathematically:

i=15i\sum_{i=1} ^{5} i

== 55 ++ i=14i\sum_{i=1} ^{4} i

== 55 ++ 44 ++ i=13\sum_{i=1} ^{3}

== 55 ++ 44 ++ 33 ++ i=12\sum_{i=1} ^{2}

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=5+4+3+2+1=5+4+3+2+1

Generic Mathematical Notation

i=1ni\sum_{i=1} ^{n} i

== nn ++ i=1n1i\sum_{i=1} ^{n-1} i

== nn ++ (n1)(n-1) ++ i=1n2\sum_{i=1} ^{n-2}

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== nn ++ (n1)(n-1) +(n2)+(n-2) $ … +2+1$

Implementation

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