In an electric circuit, three resistors with resistances r1, r2 and r3 are connected in parallel. In this case, if R is the combined resistance of these three resistors, then the reciprocal of R is equal to the sum of the reciprocals of r1, r2 and r3. What is R in terms of r1, r2 and r3?  Choose the correct choice from the following choices
A) (r1 + r2 + r3)  B) r1r2r3  C) 1/(x + y)  D) r1r2r3/(r1 + r2 + r3)  E) (r1 + r2 + r3)/r1r2r3
The wording is a bit confusing, though basically we are told that 1/R = 1/r1 + 1/r2 + 1/r3, from which it follows that r=r1r2r3/(r1 + r2 + r3).   Answer: r1r2r3/(r1 + r2 + r3).
<result>D</result>
In an electric circuit, three resistors with resistances r1, r2 and r3 are connected in parallel. In this case, if R is the combined resistance of these three resistors, then the reciprocal of R is equal to the sum of the reciprocals of r1, r2 and r3. What is R in terms of r1, r2 and r3?