If `sumn=210`, then `sumn^2=`

Explanation

Given `sumn=210`, and we have to find `sumn^2`

As
`sumn=(n(n+1))/2` then

`\Rightarrow (n(n+1))/2=210`

`\Rightarrow n^2+n-420=0`

`\Rightarrow (n-20)(n+21)=0`

`\Rightarrow n=20`

Then `sumn^2=(n(n+1)(2n+))/6`

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