Question:The mean of `1^2,2^2,3^2,4^2,5^2,6^2,7^2` is: 

A `40` 
B `20` 
C `30` 
D `10` 

+ Answer
B 

+ Explanation
`1^2 + 2^2 + 3^2 +....+n^2`

`= (n(n + 1)(2n + 1))/(6)`

`:. 1^2 + 2^2 +3^2 +......+7^2`

`= ((7 xx 8 xx 15)/6)`

`= 140`.

:. Required average

`= (140/7)`

`= 20`

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Irin Moni

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