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9.3: Cross-Sections and Nets

  • Page ID
    2168
  • Cross-sections are intersections of a plane with a solid. Nets are unfolded, flat representations of the sides of a 3-D shape that allow you to find the sum of the area of the faces by showing the edges and faces in 2-D.

    While our world is three dimensional, we are used to modeling and thinking about three dimensional objects on paper (in two dimensions). There are a few common ways to help think about three dimensions in two dimensions. One way to “view” a three-dimensional figure in a two-dimensional plane (like on a piece of paper or a computer screen) is to use cross-sections. Another way to “view” a three-dimensional figure in a two-dimensional plane is to use a net.

    Cross-Section: The intersection of a plane with a solid.

    The cross-section of the peach plane and the tetrahedron is a triangle.


    f-d_de155660fb1a2147535fabe9465761b3006179471c2c27328f22182b+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{1}\)

    Net: An unfolded, flat representation of the sides of a three-dimensional shape.


    f-d_a51b65c3a2a071621aa19ebc434ce7c7ca24702189a2ec774840b745+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{2}\)

    It is good to be able to visualize cross sections and nets as the three dimensional objects they represent.

    What if you were given a three-dimensional figure like a pyramid and you wanted to know what that figure would look like in two dimensions? What would a flat slice or an unfolded flat representation of that solid look like?

    Example \(\PageIndex{1}\)

    Describe the cross section formed by the intersection of the plane and the solid.

    f-d_2bb4389ac45c029fa09ef3af4b3131efb297be5717d64e13e975d9a3+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{3}\)

    Solution

    Circle

    Example \(\PageIndex{2}\)

    Determine what shape is formed by the following net.


    f-d_862ff82b29afe0750816e5e166520a845ef591b97fd680a72cffb56d+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{4}\)

    Solution

    Square-based pyramid

    Example \(\PageIndex{3}\)

    What is the shape formed by the intersection of the plane and the regular octahedron?


    1. f-d_8e93846c713c5b9082c49d55de17bfd16eeb7df753e9d69f69374d55+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{5}\)

    2. f-d_d5339cf61c44c4d336ae4e11260656445311492d50b2c3992af07dad+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{6}\)

    3. f-d_32d675199831a816c0646ebeb94939e7be316deffde2737f1c5cd8cb+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{7}\

    Solution

    1. Square
    2. Rhombus
    3. Hexagon

    Example \(\PageIndex{4}\)

    What kind of figure does this net create?


    f-d_af16ec0a3296682f48dc8ba87b6e45de0222d010f7c55dd5ddfd2f9a+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{8}\)

    Solution

    The net creates a rectangular prism.

    f-d_6f58a3da0d464818285b7d4a13823b219ffbc0ec5aa4b54b2c6e414b+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{9}\)

    Example \(\PageIndex{5}\)

    Draw a net of the right triangular prism below.


    f-d_fdc3134651ed30b8276bc5bfc41fa0acb5abbaed273cac8668289ff0+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{10}\)

    Solution

    The net will have two triangles and three rectangles. The rectangles are different sizes and the two triangles are the same.

    f-d_ffd1a065344de539250340ec321023c9ee55a50d2f3f6f0dc2bf7709+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{11}\)

    There are several different nets of any polyhedron. For example, this net could have the triangles anywhere along the top or bottom of the three rectangles.

    Review

    Describe the cross section formed by the intersection of the plane and the solid.


    1. f-d_e48d0d26bd7bf9ec746664d2e2e4fc638cc4c781267101710a0becab+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{12}\)
    2. f-d_bbbcbb3bc2926971c512bb616308b7213d41ccfe27b4aab32063d153+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{13}\)

    Draw the net for the following solids.


    1. f-d_725bc7b351335e7c2349ce561e8fa053188d3feef7b73043a4a7995c+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{14}\)
    2. f-d_ee3774216cde8aa76c73e73e291871201f1d3d56f723121c811f7f94+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{15}\)
    3. f-d_7429bdd976bbddcdeb97a98909fd39b126675661dd7e594ac4d171f0+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{16}\)
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      Figure \(\PageIndex{17}\)
    5. f-d_8bd37c5673795ba1cbb83690b3091849fdac40e47c28557e54774582+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{18}\)
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      Figure \(\PageIndex{19}\)

    Determine what shape is formed by the following nets.


    1. f-d_623a048c9e86e1123a934a31d2317e9fdbe6db5fdcb130fb1f7e24da+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{20}\)

    2. f-d_701215501d5b9077d34482d92874afada5b7cec731a6a67e7990fce5+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{21}\)

    3. f-d_3885f6ea4ae2c8693c9f8cee5135f1da068c7843e967dee245f56e33+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{22}\)

    4. f-d_c6e1bb0ee10289eed472c3ec9eb7467e6bd2a6a6f70ff6c5d857de1a+IMAGE_TINY+IMAGE_TINY.png
      Figure \(\PageIndex{23}\

    Review (Answers)

    To see the Review answers, open this PDF file and look for section 11.2.

    Vocabulary

    Term Definition
    cross-section The intersection of a plane with a solid.
    net An unfolded, flat representation of the sides of a three-dimensional shape.
    cross section A cross section is the intersection of a three-dimensional solid with a plane.
    Polyhedron A polyhedron is a solid with no curves surfaces or edges. All faces are polygons and all edges are line segments.
    Volume Volume is the amount of space inside the bounds of a three-dimensional object.
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