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Smaller and larger resistance in circuit

  1. Mar 1, 2010 #1
    1. The problem statement, all variables and given/known data

    When resistors 1 and 2 are connected in series, the equivalent resistance is 14.7 Ω. When they are connected in parallel, the equivalent resistance is 2.56 Ω. What are (a) the smaller resistance and (b) the larger resistance of these two resistors?

    2. Relevant equations

    1/Req=1/R1+1/R2 parallel
    R1+R2+Req series

    3. The attempt at a solution
    My t.a. said that there would be a substitution and then end up with a quadratic equation. So far i have: R1+R2=14.7ohm
    1/R1+1/R2=1/2.56ohm
    R1(R2)=2.56(R1+R2) => R1(R2)=37.632ohm
    2.56R1+2.56R2-37.632=0 <= my incorrect quadratic which i don't know how to correct.

    How can i develop a quadratic from my info? Thanks!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 1, 2010 #2

    collinsmark

    User Avatar
    Homework Helper
    Gold Member

    Hello nothingatall,

    Okay, I follow you up to here. So far things seem okay. But you've missed one of the substitutions. what you'd like to do is get an equation that is completely in terms of either R1 or R2. (See more below.)

    Nothing particularly bad, but you're failing to make a substitution. You know that

    R1+R2 = 14.7 Ohm.

    That means you also know that

    R2 = 14.7 Ohm - R1.

    Using that knowledge, now go back to on of your earlier equations (involving the equivalent parallel resistance) and try to put everything in terms of R1. Then solve for R1. Once you've found R1, you can use the above equation to get R2.
     
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