# Calculate the sum of odd numbers between 1 and 2001

Calculate the sum of odd numbers between 1 and 2001
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Frida King
How many odd numbers are between 1 and 2001?
They form a sequence which general formula is
${a}_{n}=a+\left(n-1\right)\cdot d⇒1999=3+\left(n-1\right)\cdot 2⇒n=999$
Their sum is
$S=\frac{n}{2}\cdot \left({a}_{1}+{a}_{n}\right)=\frac{999}{2}\cdot \left(3+1999\right)⇒S=999999$