1.

find sum of `1+3x+6x^2+10x^3+15x^4 + -------oo` where ` |x| < 1 , x!= 0`A. `(1)/((1-x)^(2))`B. `(1)/(1-x)`C. `(1)/((1+x)^(2))`D. `(1)/((1-x)^(3))`

Answer» Correct Answer - D
Clearly, 1,3,6,10, . . . . . Is not an A.P. But, successive differences of terms form on A.P.
Let `S=1+3x+6x^(2)+10x^(3)+15x^(4)+ . . . .oo`
`rArr" "S-xS=1+2x+3x^(2)+4x^(3)+5x^(4)+ . . . . oo` . . .(i)
`rArr" "x(S-xS)=x+2x^(2)+3x^(3)+4x^(4)+ . . .` . . .(ii)
Subtracting (ii) and (i), we get
`S(1-x)^(2)=1+x+x^(2)+x^(3)+ . . ."to "oo`
`rArr" "S(1-x)^(2)=(1)/(1-x)rArrS=(1)/((1-x)^(3))`


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