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Homework Help: Write an equation for the circle

  1. May 7, 2006 #1

    Aya

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    Hi, I need help with these questions

    Given the endpoints of a diameter, write an equation for the circle.
    a) (-3,5) and (1,3)

    I tryed this

    m= (x1+x2)/2, (y1+y2)/2
    = (-3+1)/2 , (5+3)/2
    = -2/2, 8/2
    = (-1,4)

    ( x-h)^2 + (y-k)^2=r^2
    (x+1)^2 + (y-4)^2=r^2
    (1+1)^2 + (3-4)^2=r^2
    (2)^2 + (-1)^2=r^2
    5=r^2

    so, (x+1)^2 + (y-4)^2 = 5

    but, this answer in the back of the book is different so can someone tell me what to do?

    and

    1. how do you simplify root 50
    2. is rise y and run x?

    ***
    when a stone is dropped in a lake, circular waves or ripples are created. The enrey point of the stone marks the center of the circles. Suppose the radious increases at the reate of 20cm/s. write anequation for the outermost circle 2.5s after the stone hits the water.

    20*2.5=50cm
    X^2 + Y^2=50^2
    X^2 + Y^2=2500

    is that right?
     
    Last edited: May 7, 2006
  2. jcsd
  3. May 7, 2006 #2

    dav2008

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    Gold Member

    That answer looks right to me...What is the answer that is given?

    1) To simplify the square root of 50, can you think of a way to write 50 as the product of two numbers, one of which is a perfect square?

    2) Yes. You have to be careful though because it really depends on how you set up your axis. There's nothing stopping you from writing a line in the form of x=my+a, in which case the "rise over run" is a bit misleading if you set up your axis as y vertically and x horizontally.
     
    Last edited: May 7, 2006
  4. May 7, 2006 #3

    Aya

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    ^ thanks

    I thought the two numbers were 25 and ...but how do you rght that? 2root25
    simplify es to 2root5, but thats wrong right?

    the answer in the book was x^2+y^2=49
     
  5. May 7, 2006 #4

    dav2008

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    Gold Member

    Are you sure you're checking the answer for the right problem?

    You are right that you can write 50 as 25*2

    [tex]\sqrt{50}[/tex]
    [tex]\sqrt{25\cdot 2}[/tex]

    Can you see what to do from there?
     
  6. May 7, 2006 #5

    Aya

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    ya, the book is old maby that answer is wrong.

    root 25 is 5 soo..

    5root2?
     
  7. May 7, 2006 #6

    dav2008

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    Gold Member

    Yep, that's correct.
     
  8. May 7, 2006 #7

    Aya

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    k thanks for you help
     
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