What is the value of 1+2+3+4+5+...+998+999+1000?
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here we have to implicate the below formula ∑ n = 2 n ( n + 1 ) = > ∑ n = 2 1 0 0 0 ( 1 0 0 0 + 1 ) = > ∑ n = 5 0 0 × 1 0 0 1 = > ∑ = 5 0 0 5 0 0 hence proved
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
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2 1 0 0 0 2 + 1 0 0 0 = 5 0 5 0