Find this?

The infinite circuit shown above is formed by repetition of a same link consisting of R 1 = 4 Ω R_1 = 4 \ \Omega and R 2 = 3 Ω R_2 = 3 \ \Omega .

Find the resistance of the circuit between A A and B B .


The answer is 6.

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2 solutions

Chew-Seong Cheong
Feb 28, 2019

Let the resistance of the infinite circuit between A A and B B be R R . Then the circuit is equivalent the circuit below where

R = R 1 + R 2 R = R 1 + R R 2 R + R 2 Rearranging ( R R 1 ) ( R + R 2 ) = R R 2 R 2 R 1 R R 1 R 2 = 0 Putting in R 1 = 4 , R 2 = 3 R 2 4 R 12 = 0 ( R 6 ) ( R + 2 ) = 0 R = 6 Ω Since R > 0 \begin{aligned} R = R_1 + R_2 || R & = R_1 + \frac {RR_2}{R+R_2} & \small \color{#3D99F6} \text{Rearranging} \\ (R-R_1)(R+R_2) & = RR_2 \\ R^2 - R_1R - R_1R_2 & = 0 & \small \color{#3D99F6} \text{Putting in }R_1 = 4, R_2 = 3 \\ R^2 - 4R - 12 & = 0 \\ (R-6)(R+2) & = 0 \\ \implies R & = \boxed 6 \ \Omega & \small \color{#3D99F6} \text{Since }R > 0 \end{aligned}

Good solution,easy solution .

Nnsv Abhiram - 2 years, 3 months ago

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General solution too, The same can be applied to other simple infinite circuits.

Chew-Seong Cheong - 2 years, 3 months ago

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@Nnsv Abhiram @Chew-Seong Cheong yes .it is applied for other infinite circuites. these method is used in other problems related to number theory....etc

chakravarthy b - 2 years, 3 months ago
Nnsv Abhiram
Feb 28, 2019

I AM INSPIRED BY IRODOV .SO THERE IS SPECIAL FORMULA FOR THIS .... So anwer is 6.

@Nnsv Abhiram , you should not use all cap in text. All cap in text is equivalent to shouting in voice which is rude.

Chew-Seong Cheong - 2 years, 3 months ago

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Sorry for using

Nnsv Abhiram - 2 years, 3 months ago

I have redone the image and wording for you.

Chew-Seong Cheong - 2 years, 3 months ago

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Thanks for doing

Nnsv Abhiram - 2 years, 3 months ago

How did you change the image?,And how did you type the ohm symbol.?

Nnsv Abhiram - 2 years, 3 months ago

I drew the image using Print, saved it and posted it here. You can enter LaTex code by keying in \ ( [formulas] \ ) (no space between backslash \ and the brackets ()). In the LaTex backslash-brackets type \Omega Ω \Omega . \alpha α \alpha , \beta β \beta , \gamma γ \gamma ....

Chew-Seong Cheong - 2 years, 3 months ago

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Thanks for telling.

Nnsv Abhiram - 2 years, 3 months ago

@Nnsv Abhiram @Chew-Seong Cheong do anyone know the derivation of this formula?

chakravarthy b - 2 years, 3 months ago

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Obviously it is from the third line by solving the following quadratic equation for R R .

R 2 R 1 R R 1 R 2 = 0 R = R 1 + R 1 2 + 4 R 1 R 2 2 = R 1 2 ( 1 + 1 + 4 ( R 2 R 1 ) ) \begin{aligned} R^2 - R_1R - R_1R_2 & = 0 \\ \implies R & = \frac {R_1 + \sqrt{R_1^2 + 4R_1R_2}}2 \\ & = \frac {R_1}2 \left(1 + \sqrt{1+4 \left(\frac{R_2}{R_1}\right)}\right) \end{aligned}

Chew-Seong Cheong - 2 years, 3 months ago

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@Chew-Seong Cheong Oh!! I didn't recognize it. Thanks

chakravarthy b - 2 years, 3 months ago

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