As simple as it looks

Algebra Level 2

If the sum of first N natural numbers is 210 find out the value of N


The answer is 20.

Aman Sharma by Aman Sharma , 25, India

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4 solutions

Sum = N(N+1)/2.= 210....>N=20 0r -21. Natural number. So it is 20 ~\\~\boxed{20}

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Ritam Baidya
Dec 3, 2014

the required sum is N(N+1)/2=210 N=20,-21 SO N=20 only

2 Helpful 0 Interesting 0 Brilliant 0 Confused
Azadali Jivani
Jul 17, 2015

n(1+n)/2 = 210
n(1+n) = 420
n^2 + n -420 = 0
n^2 + 21n -20n -420 = 0
n = -21 or 20 (Ans.)



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Devesh Golwalkar
Aug 14, 2014

other way to solve- first 10 natural numbers add upto 55 i.e 1+2+3+4.....10=55 similarly 11+12+1+3+14+15......+20=155 as 55+155 = 20

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