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Is cone P similar to cone Q?

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Joined: 26 Feb 2017
Posts: 17

Kudos [?]: 6 [0], given: 25

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Is cone P similar to cone Q? [#permalink]

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New post 04 Oct 2017, 10:27
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Question Stats:

29% (02:04) correct 71% (00:32) wrong based on 21 sessions

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Is cone P similar to cone Q?
(1) The surface area of P is 9 times the surface area of Q
(2) The volume of P is 9 times the volume of Q
[Reveal] Spoiler: OA

Kudos [?]: 6 [0], given: 25

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Joined: 02 Aug 2009
Posts: 4961

Kudos [?]: 5447 [1], given: 112

Is cone P similar to cone Q? [#permalink]

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New post 06 Oct 2017, 07:55
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vitorpteixeira wrote:
Is cone P similar to cone Q?
(1) The surface area of P is 9 times the surface area of Q
(2) The volume of P is 9 times the volume of Q



HI..

with 100% answering incorrectly, this may help

thing to remember..
Two cones will be similar when the RADIUS and the HEIGHT are in SAME proportion
so \(\frac{R_P}{R_Q}=\frac{H_P}{H_Q}=k\)

let's see the statements

(1) The surface area of P is 9 times the surface area of Q
this means \(9*pi*R_P^2+pi*R_P*\sqrt{H_P^2+R_P^2}=pi*R_Q^2+pi*R_Q*\sqrt{H_Q^2+R_Q^2}\)..
so \(9*=pi*R_P^2+pi*R_P*\sqrt{H_P^2+R_P^2}/pi*R_Q^2+pi*R_Q*\sqrt{H_Q^2+R_Q^2}=(R_P^2+R_P*\sqrt{H_P^2+R_P^2})/(R_Q^2+R_Q*\sqrt{H_Q^2+R_Q^2})\)
two cases..
if similar, the ratio \(k = \sqrt{9}=3\)... Possible
or it can be various combination to get the ratio as 9...Possible
Insuff

(2) The volume of P is 9 times the volume of Q
\(\frac{1}{3}*pi*R_P^2H_P=9*\frac{1}{3}*pi*R_Q^2H_Q............\frac{R_P^2H_P}{R_Q^2H_Q}=9\)..
two cases
1) if \(k^3=9\) .. similar possible
2) only \(R_P=3*R_Q\)... not similar.. possible
so insuff..

combined..
different values of k in both cases so the cones are NOT similar
Sufficient

C
_________________

Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

Kudos [?]: 5447 [1], given: 112

Is cone P similar to cone Q?   [#permalink] 06 Oct 2017, 07:55
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