Aptitude - Numbers - Discussion

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"Actions speak louder than words."

- (Proverb)

53.

(11² + 12² + 13² + ... + 20²) = ?

[A].	385	[B].	2485
[C].	2870	[D].	3255

Answer: Option B

Explanation:

(11² + 12² + 13² + ... + 20²) = (1² + 2² + 3² + ... + 20²) - (1² + 2² + 3² + ... + 10²)

Ref: (1² + 2² + 3² + ... + n²) = 1 n(n + 1)(2n + 1)

6

=		20 x 21 x 41	-	10 x 11 x 21
		6		6

= (2870 - 385)

= 2485.

Sara said: (Tue, Mar 8, 2011 10:01:25 PM)

I can't understand. Can anyone explain?

Rajni Saini said: (Fri, Mar 18, 2011 03:04:03 AM)

I can't understand, can you give any shortcut method to solve this problem plzzzz.

Jitu said: (Wed, Mar 23, 2011 08:46:50 AM)

i also not got it...... even in formula the no. is divided by 2

Varun said: (Fri, Apr 8, 2011 04:43:30 AM)

Its simple sum of squares of natural number till 20 minus sum till 10. Formula to calculate such sum is {n(n + 1)(2n + 1)}/6. n is the lats digit ie. 20 and 10 in this example