1. This forum is for support and discussion only. Please promote test prep materials/services (including AMAs) in the Special Offers subforum only. Thanks!
    Dismiss Notice
  2. Guest, be sure to check out How To Get Into Dental School, our free downloadable PDF with step-by-step details for dental school applicants!
    Dismiss Notice

crack dat math question

Discussion in 'DAT Discussions' started by msu08, Jan 2, 2009.

  1. msu08

    7+ Year Member

    Joined:
    Nov 14, 2006
    Messages:
    60
    Likes Received:
    0
    Status:
    Pre-Dental
    if x+y=3 and x^2+y^2= 8 than what is the value of xy?
    the answer is .5 and the explanation they give makes absolutely no sense to me. any ideas??
     
  2. Streetwolf

    Streetwolf Ultra Senior Member
    Dentist 7+ Year Member

    Joined:
    Oct 25, 2006
    Messages:
    1,801
    Likes Received:
    6
    Status:
    Dentist
    if x+y = 3 then (x+y)^2 = 3^2 = 9. Since (x+y)^2 is really x^2 + 2xy + y^2, then x^2 + 2xy + y^2 = 9. When you rearrange the terms on the left, you have x^2 + y^2 + 2xy = 9.

    We also know that x^2 + y^2 = 8 (given to us by the problem). So we can substitute this into our equation above:

    8 + 2xy = 9

    Thus 2xy = 1 and so xy = 1/2.
     
  3. suhshi

    7+ Year Member

    Joined:
    Dec 15, 2008
    Messages:
    481
    Likes Received:
    6
    Status:
    Dentist

    THANKS streetwolf
     
  4. joedat41

    5+ Year Member

    Joined:
    Sep 4, 2008
    Messages:
    109
    Likes Received:
    0
    Status:
    Pre-Dental
    i also had toruble with that problem. now it makes a lot of sense thanks.
     
  5. illbirz

    5+ Year Member

    Joined:
    Nov 28, 2007
    Messages:
    101
    Likes Received:
    0
    Status:
    Pre-Dental
    OH WOW...so u cant just solve for one variable for example X and then plug it in the other equation to get one and consequently the other. hhhmm...i c
     
  6. Streetwolf

    Streetwolf Ultra Senior Member
    Dentist 7+ Year Member

    Joined:
    Oct 25, 2006
    Messages:
    1,801
    Likes Received:
    6
    Status:
    Dentist
    No, you can. And you'd get an answer too. You'd get [3 +- sqrt(7)] / 2 (one of them is x and the other is y). When you multiply (3 + sqrt7)/2 and (3 - sqrt7)/2 together, you get 1/2.
     

Share This Page