1.

निम्न श्रेणी का n पदों तक योगफल ज्ञात कीजिए : `1^(2)+(1^(2)+2^(2))+(1^(2)+2^(2)+3^(2))+(1^(2)+2^(2)+3^(2)+4^(2))+...........`

Answer» दी गई श्रेणी का n वां पद `=1^(2)+2^(2)+3^(2)+4^(2)+.......+n^(2)`
`=(1)/(6)n(n+1)(2n+1)`
` =(1)/(6)(2n^(3)+3n^(2)+n)`
`=(1)/(3)n^(3)+(1)/(2)n^(2)+(1)/(6)n`
अब n पदों का योगफल `=(1)/(3)Sigma n^(3)+(1)/(2)n^(2)+(1)/(6)Sigman`
`=(1)/(3)[(n^(2)(n+1)^(2))/(4)]+(1)/(2)[(n(n+1)(2n+1))/(6)]+(1)/(6)[(n(n+1))/(2)]`
`=(1)/(12)[n^(2)(n+1)^(2)+n(n+1)(2n+1)+n(n+1)]`
`=(1)/(12)n(n+1)[(n^(2)+n)+(2n+1)+1]`
`=(1)/(12)n(n+1)(n^(2)+3n+2)`
`=(1)/(12)n(n+1)(n+1)(n+2)=(1)/(12)n(n+1)^(2)(n+2).`


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