# Little Gauss formula for the HP-15C

## Description

The sum of integers up to n are calculated.

I adapted a 9 step program by Torsten Manz to produce a new 7 step program. The 9 step program used the original formula found by Carl Friedrich Gauss:
``````
n       n(n + 1)
∑ k = ——————————
k=1         2
``````

This version uses the rearranged sum (n2 + n)/2.

g LSTx recovers n after g .

1. Enter the number n to compute the sum of integers from 1 to n.
2. Press GSB A to run.

Example:
n = 100, (n2 + n)/2 = 5050.
n= 1000, (n2 + n)/2 = 500500.

Note that the sum is the mid-point (average) x n.
For the even number sum 1 to 10, the mid-point (average) is 5.5. The sum is 5.5 x 10 = 55.
Looking at your open hands there are usually 10 fingers - assume they represent the integers 1 to 10.
Starting to count at one thumb and ending with 10 at the other thumb, the mid-point (average) is between the little fingers, i.e at 5.5.
For the odd number sum 1 to 9, the mid-point (average) is 5. The sum is 5 x 9 = 45.
For 1 to 100 you can just average 1 and 100 or 50 and 51 = 50.5. The sum is 50.5 x 100 = 5050.
For 1 to 99 the average is 50. The sum is 50 x 99 = 4950.

Sum from m to n

The generalised formula for the sum of integers from m to n is ∑x = (n(n + 1) - m(m - 1) / 2.
Setting m to zero leaves the original Little Gauss formula.
Summing from 10 to 20 includes the eleven integers from 10 to 20.
The sum is 165.

An alternative version of the formula is ∑x = (n(n + 1) - (m(m + 1))/2 + m.
Again setting m to zero gives the original Little Gauss formula.
To fully understand this look up "fencepost error" or "off by one error".

Run the Little Gauss program, for n = 20 and for m = 10.
Subtract the results to give a difference of 155.
Add m = 10, as in the alternative formula above, to give a correct result of 165.

## Program Resources

### Labels

Name Description
A Little Gauss formula

## Program

Line Display Key Sequence
000
001 42,21,11 f LBL A
002 43 11 g
003 43 36 g LSTΧ
004 40 +
005 2 2
006 10 ÷
007 43 32 g RTN