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Math Help - Help with two column proof of a trapezoid

  1. #1
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    Help with two column proof of a trapezoid

    I have been asked to do a two column proof for a trapezoid. I am stumped on what to do. I am homeschooled and I am using a tough curriculum so the book I have for geometry assumes you know how to do everything or figure it out on your own. I would really appriciate some help please!!

    figure,


    question: Given: ROSE is a trapezoid with bases OS and RE.
    Prove, OI RI = EI SI

    and this is supposed to be a two column proof.

    Thank you very very much!!
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  2. #2
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    Quote Originally Posted by Minniemouse20 View Post
    I have been asked to do a two column proof for a trapezoid. I am stumped on what to do. I am homeschooled and I am using a tough curriculum so the book I have for geometry assumes you know how to do everything or figure it out on your own. I would really appriciate some help please!!

    figure,


    question: Given: ROSE is a trapezoid with bases OS and RE.
    Prove, OI RI = EI SI

    and this is supposed to be a two column proof.

    Thank you very very much!!
    You could begin by saying that the areas of triangles OSR and SOE are equal
    as they have the same base and perpendicular heights.

    This means that triangles OIR and SIE also have equal areas as they differ from the above-mentioned triangles
    by the area of triangle OIS.

    Now write the area of the triangle OIR using the angle at I

    Area\ OIR=\frac{1}{2}|OI||IR|Sin(OIR)

    Do the same for triangle SIE

    Area\ SIE=\frac{1}{2}|SI||IE|Sin(SIE)

    However, those angles are identical.

    Hence the result follows
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  3. #3
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    two column proof of a trapezoid

    Quote Originally Posted by Archie Meade View Post
    You could begin by saying that the areas of triangles OSR and SOE are equal
    as they have the same base and perpendicular heights.

    This means that triangles OIR and SIE also have equal areas as they differ from the above-mentioned triangles
    by the area of triangle OIS.

    Now write the area of the triangle OIR using the angle at I

    Area\ OIR=\frac{1}{2}|OI||IR|Sin(OIR)

    Do the same for triangle SIE

    Area\ SIE=\frac{1}{2}|SI||IE|Sin(SIE)

    However, those angles are identical.

    Hence the result follows
    Hello Archie and Minniemouse 2,

    Archieyour proof is based on trigonometry and I believe would not be expected of a geometry student. It is aproblem involving parallel lines, transversals, equal angles and similar triangles.
    Minnie if you do not understand these clues ask for more help.

    bjh
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  4. #4
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    Quote Originally Posted by Minniemouse20 View Post
    I have been asked to do a two column proof for a trapezoid. I am stumped on what to do. I am homeschooled and I am using a tough curriculum so the book I have for geometry assumes you know how to do everything or figure it out on your own. I would really appriciate some help please!!

    figure,


    question: Given: ROSE is a trapezoid with bases OS and RE.
    Prove, OI • RI = EI • SI

    and this is supposed to be a two column proof.

    Thank you very very much!!
    If a purely geometric solution is required,
    you can use the following method....

    Side OS is parallel to side RE.

    |\angle{RIE}|=|\angle{OIS}|

    |\angle{OSR|}=|\angle{OSI}|=|\angle{SRE}|=|\angle{  IRE}|

    |\angle{SOE}|=|\angle{SOI}|=|\angle{REO}|=|\angle{  REI}|

    Hence, triangles OSI and REI are equiangular.

    Therefore triangle REI is a magnified version of triangle OSI.

    Hence if we turn triangle OSI upside-down, we can compare corresponding sides, if we also turn it back to front.

    Hence side OI corresponds to IE, SI corresponds to IR and SO corresponds to RE.

    Each side of OSI is magnified by the same scale factor to become the corresponding side of triangle REI.

    m|OI|=|IE|\ \Rightarrow\ m=\frac{|IE|}{|OI|}

    m|SI|=|IR|\ \Rightarrow\ m=\frac{|IR|}{|SI|}

    m|OS|=|RE|\ \Rightarrow\ m=\frac{|RE|}{|OS|}

    \frac{|IE|}{|OI|}=\frac{|IR|}{|SI|}\ \Rightarrow\ |IE||SI|=|OI||IR|
    Last edited by Archie Meade; June 1st 2010 at 07:52 AM. Reason: improved text
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