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Hook If you were to slice a cereal box with a geometric plane as shown, what shape would result? Coach’s Commentary I chose this example because it gives students a familiar object to envision.

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Objective In this lesson you will learn how to describe the cross sections of a right rectangular prism by slicing at different angles.

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Let’s Review Review geometric figures: right rectangular prism, plane. Coach’s Commentary Proper use of vocabulary is essential as students move to higher levels of mathematics.

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A Common Mistake A common mistake is interpreting a perspective drawing as though you were looking at it straight on. In this case, the top face of this right rectangular prism appears to be a parallelogram. However, if we stand it up and view it straight on, we can see that it is a rectangle. By keeping this in mind, we can begin to visualize more complex three-dimensional images. Coach’s Commentary Visualizing three-dimensional objects is difficult for some students. You may have to supplement this lesson with some physical models.

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**Vocabulary: Cross section**

The two-dimensional shape that results from cutting a three-dimensional shape with a plane Core Lesson Imagine the surface of the water in a cylindrical drinking glass. When we look at it from the side, the cross section appears to be an oval, but if we look at it straight on, we can see that the cross section of a cylinder is a circle. We can imagine cross sections of right rectangular prisms in a similar way. Coach’s Commentary A familiar situation such as this can help students move to more complex images.

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Core Lesson Parallel planes – two planes that never intersect, like the floor and the ceiling of a room.

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Core Lesson Perpendicular planes – two planes that intersect to form a right (90 degree) angle, like the floor and one wall of a room.

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Core Lesson If we were to cut a right rectangular prism with a plane parallel to its base, what would be the shape of the resulting cross section? Observe how the plane moves across the prism and try to imagine the resulting cross section. Notice the blue shaded region – this is the cross section. From this perspective, it looks like a parallelogram, but if we stand it up and look at it straight on, we can see that it is a rectangle. Coach’s Commentary This series of animations was designed to help students visualize the “cutting” process step by step.

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Core Lesson Now let’s consider a plane that still intersects the left and right faces of the prism, but is not parallel to the base. What shape do you think this cross section will have? Again we see the plane intersecting with the prism. Can you visualize the cross section? Now we can see the cross section in blue. Again, it looks like a parallelogram, but if we stand it up and look at it straight on, we see that once again we have a rectangle.

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Core Lesson Now let’s consider a plane that is perpendicular to the base and parallel to the front face of the prism. What shape do you think this cross section will have? Again we see the plane intersecting with the prism. Can you visualize the cross section? Now we can see the cross section in blue. This cross section is also a rectangle.

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Review In this lesson you have learned how to describe the cross sections of a right rectangular prism by slicing at different angles.

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Guided Practice Describe the shape of the cross section formed by the intersection of this plane and right rectangular prism. Answer: Observe how the plane moves through the prism. Try to visualize the resulting cross section. We can draw it in and then rotate it to look at it straight on. Another rectangular cross section is the result.

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Extension Activities For a struggling student who needs more practice: Explain why any plane parallel to one face of a right rectangular prism will intersect the prism to form a rectangular cross section. Coach’s Commentary Explaining why this happens will help students to solidify their understanding. It may be helpful to provide the student with a physical model.

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Extension Activities For a student who gets it and is ready to be challenged further: Describe the shape of the cross section formed by the intersection of a right rectangular prism and a plane that cuts off one corner of the prism.

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Answer: A rectangle parallel to the top and bottom faces.

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Answer: 2. The plane would intersect the top and bottom faces of the prism, but not be perpendicular to them.

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