Before Lunar New Year 2016 ends...

One common example of Chinese New Year symbolism is the red diamond-shaped 福 characters ("fortune"), which are displayed on the entrances of Chinese homes. This sign is usually seen hanging upside down, since the Chinese word 倒 ("upside down"), sounds like the Chinese word 到 ("arrive"). Therefore, it symbolizes the arrival of luck, happiness, and prosperity.

In the above circuit shaped in the Chinese character 福, the 14 tiny grey rectangles are all resistors with identical resistance 1 ohm. The effective resistance across A B AB can be expressed in simplest form as x y \frac{x}{y} where x x and y y are coprime. Find x + y x+y .

Note : The isolated red smaller rectangle in the diagram is not part of the circuit. Additionally, as you may have noticed, the break in the circuit is intended. There is no difference between the red and grey wires of the circuit.


The answer is 257.

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1 solution

Mark Hennings
Mar 5, 2016

Let us analyse this circuit piece by piece. Due to the break in the circuit in the middle of the diagram, there are two separate circuits running from A A to B B . The top one consists of 2 Ω 2\Omega and 1 Ω 1\Omega in parallel, followed by 1 Ω 1\Omega in series, for an effective resistance of ( 1 1 + 1 2 ) 1 + 1 = 5 3 Ω \big(\tfrac11 + \tfrac12\big)^{-1} + 1 = \tfrac53\Omega . See the first row in the diagram above.

The grouping of six 1 Ω 1\Omega resistors is equivalent, after some vertex collapse, to a series of two triples of 1 Ω 1\Omega resistors in parallel, and hence has effective resistance 2 ( 1 1 + 1 1 + 1 1 ) 1 = 2 3 Ω 2\big(\tfrac11+ \tfrac11 + \tfrac11\big)^{-1} = \tfrac23\Omega . See the second row in the diagram.

The bottom circuit from A A to B B can thus be represented by the circuit in the bottom-left of the diagram. Suppose that there is a current I I passing through this circuit, and that there are circulant currents I 1 I_1 and I 2 I_2 in the two loops as indicated. Let the voltages at the four key vertices be V V , V 1 V_1 , V 2 V_2 and V 3 V_3 as indicated. Then the equations V V 1 = I 1 V V 2 = I I 1 V 1 V 2 = I 1 I 2 V 1 V 3 = I 2 V 2 V 3 = 2 3 ( I I 2 ) \begin{array}{rclcrcl} V - V_1 & = & I_1 & \qquad \qquad & V - V_2 & = & I - I_1 \\ V_1 - V_2 & = & I_1 - I_2 & \qquad \qquad & V_1 - V_3 & = & I_2 \\ V_2 - V_3 & = & \tfrac23(I - I_2) \end{array} describe the potential losses across each component of the circuit. Solving these simultaneous equations gives I 1 = 10 21 I I 2 = 3 7 I V 1 = V 10 21 I V 2 = V 11 21 I V 3 = V 19 21 I \begin{array}{rclcrclcrcl} I_1 & = & \tfrac{10}{21}I & \; & I_2 & = & \tfrac37I \\ V_1 & =& V - \tfrac{10}{21}I &\;& V_2 & = & V - \tfrac{11}{21}I &\;& V_3 & = & V - \tfrac{19}{21}I \end{array} Thus the effective resistance of the lower circuit is V V 3 I = 19 21 Ω \tfrac{V-V_3}{I} = \tfrac{19}{21}\Omega .

Thus the whole circuit is equivalent to resistors of 5 3 Ω \tfrac53\Omega and 19 21 Ω \tfrac{19}{21}\Omega in parallel, for an overall effective resistance of ( 3 5 + 21 19 ) 1 = 95 162 Ω \big(\tfrac35 + \tfrac{21}{19}\big)^{-1} = \tfrac{95}{162}\Omega . This makes the answer 95 + 162 = 257 95 + 162 = \boxed{257} .

I did the last step by Delta-Star conversion as shown in the sketch above.

Niranjan Khanderia - 5 years, 2 months ago

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