Question

2. Determine the sum of the first 400 ODD numbers.​

Answer 1

Odd numbers take the form $2n-1$, where $n\ge1$ is an integer. When $n=400$, the last odd number would be 799. So we're adding

$S=1+3+5+\cdots+795+797+799$

By reversing the order of terms, we have

$S=799+797+795+\cdots+5+3+1$

and we can pair up terms in both sums at the same position to write

$2S=(1+799)+(3+797)+(5+795)+\cdots(795+5)+(797+3)+(799+1)$

so that we are basically adding 400 copies of 800, and from there we can find the value of the sum right away:

$2S=400\cdot800\implies S=160,000$

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We could also make use of the formulas,

$\displaystyle\sum_{i=1}^n1=n$

$\displaystyle\sum_{i=1}^ni=\dfrac{n(n+1)}2$

We have

$S=\displaystyle\sum_{i=1}^{400}(2i-1)=2\sum_{i=1}^{400}i-\sum_{i=1}^{400}1=400(400+1)-400=400^2=160,000$