1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Adding fractions and evaluating functions

  1. Feb 8, 2010 #1
    1. The problem statement, all variables and given/known data
    1.) (a + 3b) /3ab + (a^2 b - 4ab^2) / 5a^2b^2

    2.) n/(m^2) + 3/mn + 2/m

    3.) Given F(x) = square root of x + 9 determine F(x+h) - F(x)/h

    4.) say whether or not {(x,y) l x= y^2} is a function.

    2. Relevant equations

    3. The attempt at a solution

    im having problems to find the least common multiple of the algebric expresions in the first two problems.

    I think that on problem one the LCM is 15 a^2 b^2

    I think that on the second problem the LCM is m^2 (mn) Is this right?

    On problem 3 I dont know how to solve (square root of(x+h) +9 - square root of x+9)/ h

    How can I solve that? should I use the square root's conjugate? and if so which is it?

    Thanks a lot.
  2. jcsd
  3. Feb 8, 2010 #2
    You're correct about the LCM for problem 1. Recheck what you got for problem 2. You are very close. You have an extra term in it.
  4. Feb 8, 2010 #3


    Staff: Mentor

    That's not the least common multiple. The LCM is m2n. Notice that all three denominators divide this evenly.
    Multiply by 1 in the form of the conjugate over itself. I can't tell you any more because what you wrote is ambiguous. Is it f(x) = sqrt(x + 9) or sqrt(x) + 9?
    You didn't ask about 4, but you included it. Graph the relation. If it's a function, no two points will be on the same vertical line.
  5. Feb 8, 2010 #4
    I ve done the first 2 problems.

    On problem 3 I meant sqrt(x + 9) sorry.
    Is the conjugate of problem 3 sqrt(x + h + 9) + sqrt(x+9) ?

    on problem 4 I considered that x = y^2 could be seen as sqrt x = y which is a function however in the book it said that problem 4 wasent a function. So im a bit confused about it.
  6. Feb 8, 2010 #5


    Staff: Mentor

    For 3, multiply your expression by 1 in the form of [sqrt(x + h + 9) + sqrt(x+9)] over itself.

    For 4, x = y2 <==> y = +/-sqrt(x). Now do you understand your book's answer?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook