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1)

For a positive integer n, if the quadratic equation, x(x+1)+(x+1)(x+2)+.....+(x+¯n1)(x+n)=10n  has two consecutive integral solutions, then n is equal to


A) 12

B) 9

C) 10

D) 11

Answer:

Option D

Explanation:

Given quadratic equations is x(x+1)+(x+1)(x+2)+.....+(x+¯n1)(x+n)=10n

           (x2+x2+....+x2)+[1+3+5+...+(2n1)]x

                                                       +[1.2+2.3+...+(n1)n]=10n

   nx2+n2x+n(n21)310n=0

  x2+nx+n21310=0

    3x2+3nx+n2-31=0

   Let  α and β be the roots

   Since,  α and β are consecutive

        | α-β|=1

   ( α -β)2=1

Again   (αβ)2=(α+β)24αβ

  1=(3n3)24(n2313)

   1=n243(n231)

    3= 3n24n2+124

   n2=121

    n=± 11

    n=11              [ n>0]