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Geometry - Triangle problem

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jrymbei
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Geometry - Triangle problem [#permalink] New post   03 Oct 2011, 01:28
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A
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Difficulty:

55% (hard)

Question Stats:

60% (01:09) correct 40% (01:58) wrong based on 5 sessions
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Identical spherical balls are spread on a table top so as to form an equilateral triangle. How many balls are needed so that a side of the equilateral triangle contains n balls?

a. [n(n+1)]/2
b. n^2 - 1
c. n(n-1)
d. n!


I found the answer by feeding n=2,3... in to the options.



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A
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bagrettin
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Re: Geometry - Triangle problem [#permalink] New post   03 Oct 2011, 01:55
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You are right. If there are n balls on the side, then all balls will be as:
X
XX
XXX
XXXX
XXXXX

So, the total amount of balls is 1+2+3+...+n, which is the sum of the arithmetic progression, and obviously equals to n(n+1)/2. Hence, the answer is (A)
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Re: Geometry - Triangle problem [#permalink] New post   03 Oct 2011, 12:09
That's a good explanation. My approach was bit like hit and try to find the answer from the options
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arnoorichenna
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Re: Geometry - Triangle problem [#permalink] New post   03 Oct 2011, 19:18
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jrymbei wrote:
That's a good explanation. My approach was bit like hit and try to find the answer from the options


just sum of n numbers.
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Re: Geometry - Triangle problem [#permalink] New post   06 Oct 2011, 06:42
bagrettin wrote:
You are right. If there are n balls on the side, then all balls will be as:
X
XX
XXX
XXXX
XXXXX

So, the total amount of balls is 1+2+3+...+n, which is the sum of the arithmetic progression, and obviously equals to n(n+1)/2. Hence, the answer is (A)


Good question and good explanation. Kudos to both of you.



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This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
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Re: Geometry - Triangle problem [#permalink]
06 Oct 2011, 06:42
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