Question

Sum of first 'n' natural numbers is 55. Find 'n'. 

 10

 9

 8

 11

​

Answer 1

We know that sum of  firs n natural numbers is  1+ 2+ ... + n = n(n+1) / 2

So n (n+1)/2 = 55

     n^2 + n = 110

     n^2 +n - 110 = 0

    n^2 - 11n + 10n - 110= 0

    (n-11)(n+10) = 0

  n = 11 or n= -10

as n cannot be negative

n = 11

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