Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Solving an equation and checking solution

  1. Jan 31, 2010 #1
    1. The problem statement, all variables and given/known data

    (1/x-3)+(1/x+3)=(10/x²-9)


    2. The attempt at a solution

    (1/x-3)+(1/x+3)=(10/x²-9)

    (1/x-3)+(1/x+3)=(10/(x+3)(x-3)) - I factored the x^2-9 to make it easier to multiply by LCD.
    x+3+x-3=10 - I multipled both sides by the LCD [(x+3)(x-3)]
    2x=10 - Combined like terms
    x=5 - Divided both sides by 2.

    Am I wrong or is the book wrong for having the answer as x=2 ?
     
  2. jcsd
  3. Jan 31, 2010 #2
    Looks good. x=2 is clearly not a solution.
     
  4. Jan 31, 2010 #3

    HallsofIvy

    User Avatar
    Science Advisor

    Is the problem 1/x- 3+ 1/x+ 3= 10/x²- 9 or 1/(x-3)+ 1/(x+3)= 10/(x²- 9)? Those are quite different equations! You appear to be doing the second. If you multiply both sides by x²- 9 you get x+ 3+ x- 3= 2x= 10 so x= 5, as you say, not 2.

    And, of course, it is easy to put x= 2 into the original equation and check: 1/(2-3)+ 1/(2+3)= -1+ 1/5= (-5+ 1)/5= -4/5 which is NOT 10/(4- 9)= 10/(-5)= -2.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook