Taking it to the next level!

Identical wires, each with resistance R = 1 Ω , R = 1\, \Omega, are connected to form the edges of the Platonic solid icosahedron in the figure. Note that it is a polyhedron with 20 20 faces of equilateral triangles.

Determine the resistance of the network between the two opposite vertices A A and B . B.


Source : INPhO 1999


The answer is 0.5.

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3 solutions

Steven Chase
May 15, 2018

By symmetry, there are actually only two unique voltages in the circuit. Call these V 1 V1 and V 2 V2 . These are the voltages at the electrical nodes, which are the vertices of two regular pentagons. Write the nodal equations for a representative node from each pentagon, supposing that we apply a 1-volt DC source from B to A.

3 V 1 2 V 2 = 1 2 V 1 + 3 V 2 = 0 3 V1 - 2 V2 = 1 \\ -2 V1 + 3 V2 = 0

Solving gives V 1 = 0.6 V1 = 0.6 and V 2 = 0.4 V2 = 0.4 . The current flowing into B is:

I = 5 ( 1 V 1 ) = 5 ( 1 0.6 ) = 2 I = 5 (1 - V1) = 5 (1 - 0.6) = 2

Since we applied 1 V and got 2 amps flowing out of the source, this corresponds to a total resistance of 0.5 Ω 0.5 \Omega

Observe, that by symmetry the potentials at points G , C , D , E , F G,C,D,E,F are equal. So no current will flow from G C , C D , D E , E F , F G GC,CD,DE,EF,FG .

So we can join the points G , C , D , E , F G,C,D,E,F .

Similarly, the potentials at H , I , J , K , L H,I,J,K,L are equal. So no current will flow from H I , I J , J K , K L , L H . HI,IJ,JK,KL,LH. So these points can also be joined.

So the circuit simplifies to:- Now from A to B, the resistances are in series, so adding them gives R e q = 1 2 Ω R_{eq}=\boxed{\frac{1}{2} \Omega}

@Shreyansh Mukhopadhyay How many marks in phase test you got.Answer keys out.

Ayon Ghosh - 2 years, 10 months ago

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i checked only IIT-178, yours?

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay i am sorry bhai.right now i am admit in hospital since midnight yesterday with dengue fever.i dont have my papers to check it.maybe i will be discharged in 4-5 days.

Plz tell some controversial answers.That M'=M question.

Looking and remembering some answers.I think 250-265 is my best shot.

Ayon Ghosh - 2 years, 10 months ago

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@Ayon Ghosh (A)M'>M I'am getting 46 in MAT, 66 in SAT. And how did you get to know it was dengue? Are you serious?

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay Blood test in hospital showed dengue positive.

Ayon Ghosh - 2 years, 10 months ago

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@Ayon Ghosh plz tell answer to that Magnesite dolomite matrix match.I think i have some controversy there

Ayon Ghosh - 2 years, 10 months ago

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@Ayon Ghosh P-Carbonate ore

Q-Contains Mg

R-Contains Mg and Ca

S-Ore of Ca

A)Magnesite-P,Q

B)Dolomite-P,R

C)Lime stone-P,S

D)Calcite-P

And your pre mid marks are:-

History-19/20

Physics-13/13

Maths-40/40

And Richa ma'am told your letter to editor was best in the class

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay what are my other marks ?

Ayon Ghosh - 2 years, 10 months ago

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@Ayon Ghosh I don't remember others but in bio yours is definitely less than 12.5 /13 because highest is 12.5 (amogh)

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay i am screwed.i will be discharged on saturday.Not gonna attend school for the rest of the week.I wanna argue some answers related to phase tests.Can you plz get Shinde Sir 's number for me when you go to fiitjee on wednesday ?

Ayon Ghosh - 2 years, 10 months ago

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@Ayon Ghosh Ok will try

Shreyansh Mukhopadhyay - 2 years, 10 months ago

@Ayon Ghosh Get well soon then

Shreyansh Mukhopadhyay - 2 years, 10 months ago

what happened at fiitjee ?

Ayon Ghosh - 2 years, 10 months ago

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Answer key discussion. If you have any doubts call between 12pm and 1pm tomorrow at fiitjee office. Anyways your doubts have already been discussed.

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay What happened at fiitjee today ? ahem fine ?

Probably gonna be able to attend the next fiitjee class !!

Ayon Ghosh - 2 years, 10 months ago

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@Ayon Ghosh So are you discharged. Nothing much happened at fiitjee. We studied plane mirror, rmo quiz, vishal sir absend. And I can't tell about your ahem .

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay yes finally no.28 (my hospital name) will be discharged today !!

Ayon Ghosh - 2 years, 10 months ago

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@Ayon Ghosh Will you give aiots

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay Nopes.Havent studied anything.Probably gonna do optics from DCP now.

Ayon Ghosh - 2 years, 10 months ago

@Ayon Ghosh , congrtats you scored highest 382 excluding subjective. @Amogh Huddar -341, he scored more than shristi

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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how u know ? i am scoring 399 man, withought subs. 78+82+94(iit) (-2) SAT 84 MAT 64. answer key mistake ? who is second ?

Ayon Ghosh - 2 years, 10 months ago

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Mat-54. Iit-246 . Second is kislay-374

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay Shashank ? i saw shrishti 's result,its pretty pathetic except maths !

Ayon Ghosh - 2 years, 10 months ago

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@Ayon Ghosh 351 excluding subjective

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay Give his and amogh enrol number.

Ayon Ghosh - 2 years, 10 months ago

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@Ayon Ghosh Amough-1061461810013, and his-1061461810017

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay and some other marks - anurag,saksham ?

Ayon Ghosh - 2 years, 10 months ago

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@Ayon Ghosh My predicted rank is 17th. and amough's 4th

Shreyansh Mukhopadhyay - 2 years, 10 months ago

@Ayon Ghosh saksham-323

anurag-304

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay ok bye will talk later again.

Ayon Ghosh - 2 years, 10 months ago

@Shreyansh Mukhopadhyay so what s the predicted rank list ?

Ayon Ghosh - 2 years, 10 months ago

@Ayon Ghosh Are you going to discharge todaay

Shreyansh Mukhopadhyay - 2 years, 10 months ago

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@Shreyansh Mukhopadhyay No unfortunately.

Ayon Ghosh - 2 years, 10 months ago

@Ayon Ghosh , AIOTS score?, Anurag told you are geting 157. Seriously?

Shreyansh Mukhopadhyay - 2 years, 9 months ago
Ayon Ghosh
May 15, 2018

its hard to write a solution at 3:00 A.M. but i will try my best.

Just like in case of a cube ,here also we use symmetry to eliminate the 10 resistors in the middle are they are connected across equipotential points.We are left with with a 5-10-5 parallel series chain which one easily fets R/2 as the equivalent resistsnce.

Thanks for posting the question . How many tests are there? And was bio tough?

This also came in a past INPhO. (199x)

Arhaan Ahmad - 2 years, 5 months ago

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1999.I have the old INPhO papers from 1998-2005.

Ayon Ghosh - 2 years, 5 months ago

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Yeah. I had also found it there ;)

Arhaan Ahmad - 2 years, 5 months ago

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