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Math Help - prove

  1. #16
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    prove-equilateral.jpg
    Quote Originally Posted by yehoram View Post
    Here is a drawing of the problem !
    Due to symmetry, if angle ACE is 30 degrees, then triangle AGC is equilateral.

    If we use the corners of the square as circle centres, with radius "x",
    and using the fact that the angle at the centre of a circle is twice the angle at the circumference
    for angles standing on a common arc,

    then it follows that if angle ACE is 30, then the angles ADE and AED are 15 degrees.

    Triangle AGC is equilateral as all sides are R.
    The angles of AGC are all 4\theta, so \theta=15^o
    Attached Thumbnails Attached Thumbnails prove-equilateral2.jpg  
    Last edited by Archie Meade; August 14th 2010 at 02:35 AM.
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  2. #17
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    Maybe simplest is to solve for \beta in the diagram.
    Attached Thumbnails Attached Thumbnails prove-equi3.jpg  
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  3. #18
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    Quote Originally Posted by sa-ri-ga-ma View Post
    From the picture it is clear that angle AEM = EMQ = 15 degrees.
    The picture, however is not true, nor is it accurate. You cannot just make a statement like that.

    EDIT: Your last post Archie, that's a great solution!
    Last edited by Vlasev; August 13th 2010 at 01:01 PM.
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  4. #19
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    prove

    Hello yehoram,
    Your problem was poorly worded,corrected by diagram but failing to redefine the. givens
    At C draw an arc r=1 from Dto B and at B an arc fromCto E.These arcs meet at A along the perpendicular bisector of CB and DE
    DA is a chord in a quarter circle. EDA is given 15 DE is tangent to circle at D
    Arc DA = 30 Angle DCA =30 Angle ACB=60


    bjh
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  5. #20
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    Quote Originally Posted by Vlasev View Post
    The picture, however is not true, nor is it accurate. You cannot just make a statement like that.

    EDIT: Your last post Archie, that's a great solution!

    Which picture is not correct?

    What is wrong in my statement?

    I have drawn PAQ parallel to DME. EMAQ is a rectangle. Angle AEM = 15 degrees is given in the problem. In the rectangle EMAQ angle MEA = Angle EMQ = 15 degrees. R is the point of intersection of the sector and MA. I have drawn SRT parallel to EMD.
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  6. #21
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    Quote Originally Posted by sa-ri-ga-ma View Post
    Which picture is not correct?

    What is wrong in my statement?

    I have drawn PAQ parallel to DME. EMAQ is a rectangle. Angle AEM = 15 degrees is given in the problem. In the rectangle EMAQ angle MEA = Angle EMQ = 15 degrees. R is the point of intersection of the sector and MA. I have drawn SRT parallel to EMD.
    In the picture the angle is close to 30 degrees not 15. Look at Archie Meade's last post. That drawing is much more accurate. If your method works with it, I think it'll be fine!
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  7. #22
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    This reply might be a bit late. But why make it harder than it is .

    From your drawing

    \angle{DAE} = 180 - 15 - 15 = 150

    \angle{ADB}=\angle{AEC} = 90 - 15 = 75

    \angle{EAC}=\angle{DAB} = \beta

    If \bigtriangleup{ABC} is equilateral set all its angles to \alpha

    This gives us:
    150 + 2*\beta + \alpha = 360 \Rightarrow \alpha = 210 - 2*\beta
    75 + \beta + (90-\alpha) = 180 \Rightarrow \beta = 15 - \alpha
    \alpha = \frac{210-30}{3} = 60

    Which proves that \bigtriangleup{ABC} is equilateral.
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  8. #23
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    Quote Originally Posted by obidobi View Post
    This reply might be a bit late. But why make it harder than it is .

    From your drawing

    \angle{DAE} = 180 - 15 - 15 = 150

    \angle{ADB}=\angle{AEC} = 90 - 15 = 75

    \angle{EAC}=\angle{DAB} = \beta

    If \bigtriangleup{ABC} is equilateral set all its angles to \alpha

    This gives us:
    150 + 2*\beta + \alpha = 360 \Rightarrow \alpha = 210 - 2*\beta
    75 + \beta + (90-\alpha) = 180 \Rightarrow \beta = 15 - \alpha
    \alpha = \frac{210-30}{3} = 60

    Which proves that \bigtriangleup{ABC} is equilateral.
    I'm afraid that is circular reasoning!
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  9. #24
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    If CAD is isoceles.. the lines are diagnols dude
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  10. #25
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    thats correct.. its circular reasoning.. uve taken a preusumption that the triangle is equ. and proved it.. doesnt amount to nething..
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  11. #26
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    Quote Originally Posted by Vlasev
    I'm afraid that is circular reasoning!
    Quote Originally Posted by umangarora
    thats correct.. its circular reasoning.. uve taken a preusumption that the triangle is equ. and proved it.. doesnt amount to nething..
    Yes. I should have turned on my brain before thinking

    Oki lets do this the right way then:


    Known:
    x = a + b
    x = 2h

    Problem: Find a value for a so that r_0 = 2h = x, then left triangle is equilateral

    Pythagoras gives:
    a^2 + h^2 = r_0^2
    a^2 + (\frac{x}{2})^2 = x^2
    a = \frac{\sqrt{3}}{2}x
    b = \frac{2-\sqrt{3}}{2}x

    \tan{\alpha} = \frac{b}{h}
    \alpha = \arctan{(2-\sqrt{3})}
    \alpha = 15^{\circ}
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  12. #27
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    Quote Originally Posted by yehoram View Post
    Hi ! I have some problem with this below !

    Subjected to a square which has two isosceles triangles.
    One of them is: a triangle whose base is one of the sides of the square root angles are 15.
    The other is a triangle whose base is the side opposite the base of the triangle it first touches the apex of the triangle at the end of the first node.
    Proved that the second triangle equilateral!

    The prove must be geometric way , no trigonometric ! Please help me ! thanks all !

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