1.

If `a_1,a_2, a_3, a_4`be the coefficient of four consecutive terms in the expansion of `(1+x)^n ,`then prove that: `(a_1)/(a_1+a_2)+(a_3)/(a_3+a_4)=(2a_2)/(a_2+a_3)dot`

Answer» First four coefficient terms of `(1+x)^n` are,
`1, C(n,1), C(n,2),C(n,3)`
`:. a_1 = 1`
`a_2 = n`
`a_3 = (n(n-1))/2`
`a_4 = (n(n-1)(n-2))/6`
Now,
`L.H.S. = a_1/(a_1+a_2) + a_3/(a_3+a_4)`
`=1/(n+1) +( (n(n-1))/2)/( (n(n-1))/2+ (n(n-1)(n-2))/6)`
`=1/(n+1) +(1/(1+((n-2))/3))`
`=1/(n+1)+3/(n+1) = 4/(n+1)`
Now, `R.H.S. = (2a_2)/(a_2+a_3)`
`=(2n)/(n+(n(n-1))/2)`
`=(2*2)/(2+n-1)`
`=4/(n+1)`
`:. L.H.S. = R.H.S.`


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