Identical wires, each with resistance R = 1 Ω , are connected to form the edges of the Platonic solid icosahedron in the figure. Note that it is a polyhedron with 2 0 faces of equilateral triangles.
Determine the resistance of the network between the two opposite vertices A and B .
Source : INPhO 1999
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Observe, that by symmetry the potentials at points G , C , D , E , F are equal. So no current will flow from G C , C D , D E , E F , F G .
So we can join the points G , C , D , E , F .
Similarly, the potentials at H , I , J , K , L are equal. So no current will flow from H I , I J , J K , K L , L H . So these points can also be joined.
So the circuit simplifies to:- R e q = 2 1 Ω
Now from A to B, the resistances are in series, so adding them gives@Shreyansh Mukhopadhyay How many marks in phase test you got.Answer keys out.
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i checked only IIT-178, yours?
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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.
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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?
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@Shreyansh Mukhopadhyay – Blood test in hospital showed dengue positive.
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@Ayon Ghosh – plz tell answer to that Magnesite dolomite matrix match.I think i have some controversy there
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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
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@Shreyansh Mukhopadhyay – what are my other marks ?
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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)
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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 – Get well soon then
what happened at fiitjee ?
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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.
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@Shreyansh Mukhopadhyay What happened at fiitjee today ? ahem fine ?
Probably gonna be able to attend the next fiitjee class !!
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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 .
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@Shreyansh Mukhopadhyay – yes finally no.28 (my hospital name) will be discharged today !!
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@Ayon Ghosh – Will you give aiots
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@Shreyansh Mukhopadhyay – Nopes.Havent studied anything.Probably gonna do optics from DCP now.
@Ayon Ghosh , congrtats you scored highest 382 excluding subjective. @Amogh Huddar -341, he scored more than shristi
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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 ?
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Mat-54. Iit-246 . Second is kislay-374
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@Shreyansh Mukhopadhyay – Shashank ? i saw shrishti 's result,its pretty pathetic except maths !
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@Ayon Ghosh – 351 excluding subjective
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@Shreyansh Mukhopadhyay – Give his and amogh enrol number.
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@Ayon Ghosh – Amough-1061461810013, and his-1061461810017
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@Shreyansh Mukhopadhyay – and some other marks - anurag,saksham ?
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@Ayon Ghosh – My predicted rank is 17th. and amough's 4th
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@Shreyansh Mukhopadhyay – ok bye will talk later again.
@Shreyansh Mukhopadhyay – so what s the predicted rank list ?
@Ayon Ghosh – Are you going to discharge todaay
@Ayon Ghosh , AIOTS score?, Anurag told you are geting 157. Seriously?
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)
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1999.I have the old INPhO papers from 1998-2005.
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By symmetry, there are actually only two unique voltages in the circuit. Call these V 1 and V 2 . 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
Solving gives V 1 = 0 . 6 and V 2 = 0 . 4 . The current flowing into B is:
I = 5 ( 1 − V 1 ) = 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