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Question
Find the sum of all even integers between 101 and 999.
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Solution
In this problem, we need to find the sum of all the even numbers lying between 101 and 999.
So, we know that the first even number after 101 is 102 and the last even number before 999 is 998.
Also, all these terms will form an A.P. with the common difference of 2.
So here
First term (a) = 102
Last term (l) = 998
Common difference (d) = 2
So, here the first step is to find the total number of terms. Let us take the number of terms as n.
Now, as we know,
`a_n = a + (n - 1)d`
So, for the last term
`998= 102 + (n - 1)2`
998 = 102 + 2n - 2
998 = 100 + 2n
998 - 100 = 2n
Further simplifying
898 = 2n
`n = 898/2`
n = 449
Now using the formula for the sum of n terms
`S_n = n/2 [2a + (n - 1)d]`
For n = 64 we get
`S_n = 449/2[2(102) + (449 - 1)2]`
`= 449/2 [204 + (448)2]`
`= 449/2 (204 + 896)`
`= 449/2 (1100)`
On further simplification, we get,
`S_n = 449(550)
= 246950
Therefore the sum of all the even number lying between 101 and 999 is `S_n = 246950`
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