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Math Help - Convex help please!

  1. #1
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    Convex help please!

    Let A be a set of points. Let B be the union of all segments of the form line segment PQ, where P and Q belong to A. Does it follow that B is convex? Why or why not?

    Here's what I think:

    Yes, it follows that B is convex because if B is contained in A and P, Q are members of A the line segment PQ is in B, then B has to belong to A and the line segment PQ lies in A making B convex.

    Is this right? Can you help me if it isn't please?

    My book defines that convex as for every two points P,Q of A the entire segment PQ lies in A.

    Thanks for all of the help!
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  2. #2
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    Quote Originally Posted by rulergirl26 View Post
    Let A be a set of points. Let B be the union of all segments of the form line segment PQ, where P and Q belong to A. Does it follow that B is convex? Why or why not?

    Here's what I think:

    Yes, it follows that B is convex because if B is contained in A and P, Q are members of A the line segment PQ is in B, then B has to belong to A and the line segment PQ lies in A making B convex.

    Is this right? Can you help me if it isn't please?

    My book defines that convex as for every two points P,Q of A the entire segment PQ lies in A.

    Thanks for all of the help!
    rulergirl26,

    Hint:

    Try points A_1, A_2, A_3 which are vertices of a triangle. Then B is the triangle, consisting of 3 line segments.

    Is a triangle convex?
    Last edited by awkward; September 22nd 2008 at 07:11 PM. Reason: clarification
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  3. #3
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    Quote Originally Posted by awkward View Post
    rulergirl26,

    Hint:

    Try points A_1, A_2, A_3 which are vertices of a triangle. Then B is the triangle, consisting of 3 line segments.

    Is a triangle convex?

    Yes triangles are always convex because all line segments connecting two points on its surface lie entirely within the object.

    So, the proof would start out as:

    Given points A_1, A_2, A_3 which are vertices of a triangle. Then B is the triangle, consisting of 3 line segments...

    But how or what do I say about the line segment PQ? Do I say that since th line segment PQ belongs to A then it has to lie in B making B convex? Something like that?

    Thanks for your help!!!
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  4. #4
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    Quote Originally Posted by rulergirl26 View Post
    Yes triangles are always convex because all line segments connecting two points on its surface lie entirely within the object.

    So, the proof would start out as:

    Given points A_1, A_2, A_3 which are vertices of a triangle. Then B is the triangle, consisting of 3 line segments...

    But how or what do I say about the line segment PQ? Do I say that since th line segment PQ belongs to A then it has to lie in B making B convex? Something like that?

    Thanks for your help!!!
    Ahem. No, triangles are not convex.

    Keep in mind that a triangle consists of the three line segments. It does not include the interior of the triangle.
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