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Two resistors when connected in parallel give the resultant resistance of 2 ohm; but when connected in series the effective resistance becomes 9 ohm. Calculate the value of each resistance.

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Let the resistance be R1 and R2 when two resistances are connected in series

RS = R1 + R2

= 9

R1 + R2 = 9 ……….(1)

When two resistance are connected in parallel

Using (1) equation (2) becomes

\(\frac{9}{R_1R_2}\)

R1R2 = 18 ...... (3)

(R1 – R2)2 = (R1 + R2)2 – 4R1R2 

= (9)2 – 4 × 18 

= 81 – 72 = 9

∴ (R1 – R2 ) = √9 = 3 ………(4) 

From (1) 

R1 + R2 = 9 

2R1 =12 

∴ R1 =\(\frac{12}{2}\) = 6 ohm 

From (1) 

R2 = 9 – R1 

= 9 – 6 = 3Ω 

The values of resistances are 

R1 = 6 ohm 

R2 = 3 ohm

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