the total resistance R of two resistors with resistance r1 and r2, joined in parrallel, is given by the formula 1/R=1/r1+1/r2

There is an error in Rom’s input values.

This above formula effectively creates a harmonic mean, then divides that mean by the number of resisters.

For the values in the question R1=4 ohms and R2=3 ohms The harmonic mean is

$$\\\frac{2}{( \frac{1}{4}+ \frac{1}{3})} \ = 3.43\ ohms$$

This averages to

$$\frac{3.43}{2} = 1.71\ ohms$$

The inverse of ohm is Mho, and is a unit of conductance. This was used by Rom in his equation. A Mho is also known as a Siemen.

~~~D~~

$$\dfrac 1 R = \dfrac 1 {r1} + \dfrac 1 {r2} ~mho$$

$$\dfrac 1 R = \dfrac 1 1 + \dfrac 1 3 = \dfrac 4 3~mho$$

$$R = \dfrac 3 4~ohm$$

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Hi anonymous,

I have no idea which is correct.  As the question asker it is up to you to have enough knowledge to know which answer to accept and which to reject.

Maybe Rom or someone else will come along and help you with this task.

Rom's method is correct; he just used a value for r1 of 1 instead of the requested 4. 

1/R = 1/4 + 1/3  = 3/12 + 4/12 = 7/12

Hence R = 12/7 = 1.714 ohms

i.e. the same as DavidQD