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: Proof about symmetry (simple)

  1. Dec 23, 2011 #1
    1. The problem statement, all variables and given/known data

    Show that the curve is symmetrical about the x axis (without drawing the graph)
    eq of the curve is : x^2 + 4y^2 = 1

    also show that the curve is symmetric about the y axis
    2. Relevant equations

    3. The attempt at a solution

    To prove that the curve was symmetric abou the x axis, i made x the subject of the equation of the curve:

    x=(1-4y^2)^0.5 (can be positive or negative)

    Then i used simple intuition:
    let a particular value of y be "k" and the corresponding value of x be "c".
    by simple calculation, we can conclude that for y=-k , x will still be equal to "c"

    Can anyone guide me if this proof is enough or it lacks something, for the latter case, please provide an alternative but suitable proof.
  2. jcsd
  3. Dec 23, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Generally, simply use the original equation. If you replace y with -y, and the resulting equation is equivalent to the original equation, then the graph is symmetric w.r.t. the x-axis.

    For symmetry w,r,t, the y-axis, replace x with -x, in the original, and check to see that the result is equivalent to the original.
  4. Dec 23, 2011 #3


    User Avatar
    Homework Helper

    It is enough proof to show that f(x)=f(-x) for symmetry about the y-axis, and f(y)=f(-y) for symmetry about the x-axis. Can you see why?
    Basically, this just means you just need to show that the function doesn't change when you swap x for -x and y for -y.

    edit: If I bothered to refresh the page to see if a reply was already made, we wouldn't be here right now...
  5. Dec 23, 2011 #4
    alright, from your replies i think the method i used is correct.

    So thnx guys !
  6. Dec 24, 2011 #5


    User Avatar
    Science Advisor

    Yes, correct, but do you understand that everyone was telling you that you don't have to solve for one variable?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook