1.

`1. 2 .3+2. 3 .4++n(n+1)(n+2)=(n(n+1)(n+2)(n+3))/4`

Answer» Let P (n) `: 1.2.3+2.3.4+3.4.5+……`
`+n(n+1)(n+2)=1/4n(n+1)(n+2)(n+3)`
For n=1,
`L.H.S. =1.2.3=6`
`R.H.S. =1/4 .1.(1+1)(1+2)(1+3)=6`
`:. L.H.S. =R.H.S`
`rArr` P (n) is true for n=1
Let P (n) be true for n=k
`:. P (k) : 1.2.3 + 2.3.4+3.4.5+......`
`.......+k(k=1)(k+2)`
`=1/4 k(k+1)(k+2)(k+3)`
For n=k+1
`P (k+1) : 1.2.3+2.3.4+3.4.5+........+k(k+1)`
`(k+2)+(k+1)(k+2)(K+3)`
`=1/4k(k+1)(k+2)(K+3)+(K+1)(K+2)(K+3)`
`=(k+1)(K+2)(K+3) ((K)/(4)+1)`
`=1/4 (k+1) (k+2) (k+3) (k+4)`
`rArr` P (n) is also true for n=k+1
hence from the principle of mathematical induction P (n) is true for all natural numbers n.


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