A number theory problem by Rithvik Gundlapalli

What is the value of 1+2+3+4+5+...+998+999+1000?


The answer is 500500.

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4 solutions

Syed Hamza Khalid
May 11, 2017

100 0 2 + 1000 2 = 5050 \frac{1000^2+1000}{2}=5050

Sudoku Subbu
Jan 24, 2015

here we have to implicate the below formula n = n ( n + 1 ) 2 \sum n=\frac{n(n+1)}{2} = > n = 1000 ( 1000 + 1 ) 2 =>\sum n =\frac{1000(1000+1)}{2} = > n = 500 × 1001 =>\sum n=500 \times 1001 = > = 500500 => \sum=500500 hence proved

Lu Chee Ket
Jan 23, 2015

S = 1 + 2 + 3 + ... + 998 + 999 + 1000

S = 1000 + 999 + 998 + ... + 3 + 2 + 1

2 S = 1000 (1001)

S = 500 (1001) = 500500

When you have to do a small version of this problem, like the sum of the numbers from 1 to 20, or even 30, it's possible to do it mentally or on paper. But when it comes to the bigger end of things, you have to use valuable formulas such as Gauss's Formula for Adding Consecutive Integers from 1 to n: [n ( n+1 )]/2.

[1000 ( 1000+1 )]/2 1000 x 1001 = 1001000 1001000 / 2 = 500500

So, 500500 is your answer!

I did it mentally in 15 seconds. 1001 * 500 = 500500

Denton Young - 5 years, 9 months ago

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