1.

निम्नलिखित श्रेणी के n पदों तक योगफल ज्ञात करो- `1.2^(2)+2.3^(2)+3.4^(2)+......`

Answer» n वां पद `=n.(n+1)^(2)=n^(3)+2n^(2)+n`
अतः योगफल `=Sigma n^(3)+2Sigman^(2)+Sigma n`
`=(1)/(4)n^(2)(n+1)^(2)+(1)/(3)n(n+1)(2n+1)+(1)/(2)n(n+1)`
`=(1)/(12)n(n+1){3n(n+1)+4(2n+1)+6}`
`=(1)/(12)n(n+1){3n^(2)+11n+10}`
`=(1)/(12)n(n+1)(n+2)(3n+5).`


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