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| 13 Theory slides |
| 8 Exercises - Grade E - A |
| Each lesson is meant to take 1-2 classroom sessions |
In the diagram, a cube is given. Try to identify how many different cross-sections can be formed. What geometric shapes do these cross-sections have?
These are the possible interactions with the presented applet.
Rotateor click on the circle and move it around.
Reset.
These are possible interactions with the presented applet.
These are the possible interactions with the presented applet.
Rotatebutton.
Resetbutton.
From the previous applet, it could be concluded that rotating a rectangle about one of its sides forms a right cylinder. What are the cross-sections of a right cylinder? There are several types depending on the position of an intersecting plane.
Case | Position of the Plane | Cross-Section |
---|---|---|
1 | Perpendicular to the base | Rectangle |
2 | Parallel to the base | Circle |
3 | Diagonal to the base | Ellipse |
The following applet illustrates each type of the mentioned cross-sections.
Analyze the shapes of the cross-sections closely. What conclusions can be drawn about the sides and angles of the cross-sections?
To pair each plane with the corresponding type of cross-section, each given case should be investigated.
The first intersecting plane is parallel to the cylinder's bases. Therefore, the cross-section it creates has the same shape as the bases, which are circles.
Hence, the cross-section is also a circle.
The intersecting plane is perpendicular to both bases of the cylinder. Since the cylinder is right, the cross-section has four right angles. Additionally, the longer sides of the cross-section are parallel to the height of the cylinder and have the same lengths.
Furthermore, the shorter sides are parallel chords of the cylinder's bases, which also have equal lengths. Therefore, the cross-section is a rectangle.
The last plane intersects only the curved surface of the cylinder, so it does not have straight sides. The cross-section has a circular shape, but it is not a circle.
Recall the possible cross-sections of a right cylinder depending on the position of the intersecting plane.
The Plane's Position | Cross-Section |
---|---|
Perpendicular to the base | Rectangle |
Parallel to the base | Circle |
Diagonal to the base | Ellipse |
Since the intersecting plane is diagonal to the bases of the cylinder, it can be concluded that the cross-section is an ellipse.
The given two-dimensional shape is a cross-section of a right cylinder. Identify whether the cross-section is parallel, perpendicular, or diagonal to the base of the cylinder.
These are the possible interactions with the presented applet.
Rotate.
Reset.
From the previous applet, it can be observed that when rotating a right triangle about its height, a right cone is formed. What are the cross-sections of a right cone? Here are some possible types depending on the position of an intersecting plane.
The Plane's Position | Cross-Section |
---|---|
Perpendicular to the base | Triangle |
Parallel to the base | Circle |
Diagonal to the base | Ellipse |
Analyze the shapes of the cross-sections closely. What conclusions can be drawn about the sides and angles of the cross-sections?
To pair each plane with the corresponding type of cross-section, each given case should be analyzed.
The first intersecting plane is parallel to the cone's base. Consequently, the cross-section it creates has the same shape as the base, which is a circle.
Therefore, the cross-section is also a circle.
The second plane goes through the curved surface of the cone, so it does not have any straight sides. The cross-section has a circular shape, but it is not a circle.
To identify the cross-section, the possible cross-sections of a right cone can be reviewed.
The Plane's Position | Cross-Section |
---|---|
Perpendicular to the base | Triangle |
Parallel to the base | Circle |
Diagonal to the base | Ellipse |
It can be noticed that the intersecting plane is diagonal to the bases of the cone. Therefore, the cross-section can be concluded to be an ellipse.
The last intersecting plane is perpendicular to the base of the cone and goes through the vertex of the cone. The cross-section has three vertices, which are connected by sides.
Therefore, the cross-section is a triangle.
The given 2D shape is a cross-section of a right cone. Identify whether the cross-section is parallel, perpendicular, or diagonal to the base of the cone.
3D Object | Intersecting Plane's Position | Cross-Section |
---|---|---|
Sphere | Any position | Circle |
Right Cylinder | Perpendicular to the base | Rectangle |
Parallel to the base | Circle | |
Diagonal to the base | Ellipse | |
Right Cone | Perpendicular to the base | Triangle |
Parallel to the base | Circle | |
Diagonal to the base | Ellipse |
The following figure shows a cube with a side length of 5 inches and a cross-section through four of its vertices.
The cross-section of the cube consists of two sides and two diagonals of the cube. Let's mark them on the diagram.
Knowing that the cube has sides of 5 inches, we can find the length of the diagonal by using the Pythagorean Theorem.
Now that we know the length of the diagonal, we can find the perimeter of the cross-section by adding the lengths of the sides.
The perimeter is 10+10sqrt(2) inches.
To find the area of the rectangular cross-section, we will multiply the length and width of the cross-section.
The area is 25sqrt(2) square inches.
Consider the following cube.
Let's investigate the potential cross-sections of the cube one at the time.
Let's recall the definition of a circle.
A circle is the set of all points that are equidistant from a given point.
From this definition, we can conclude that a circle has a rounded shape. A cube, though, is a polyhedron. Regardless of how the plane intersects the cube, the resulting shape will always be a polygon. Therefore, we know that it is not possible for a cross-section of a cube to be a circle.
As we previously discussed, no matter how the plane intersects the cube, the result will always be a polygon. If the plane passes through four edges and a vertex of the cube, we have a cross-section in the shape of a pentagon.
Notice that a rhombus can be a square if all the angles are right. Therefore, we can create a rhombus with a cross-section that runs parallel any face of the cube.
An isosceles triangle is a triangle with a pair of congruent sides. If we let the plane pass through the three edges of a common vertex where two points of intersection are at the same distance from the vertex, we will get an isosceles triangle.
In conclusion, the cross-section of a cube can create a pentagon, a rhombus, and an isosceles triangle. & A Circle && * & B Pentagon&& ✓ & C Rhombus&& ✓ & D Isosceles Triangle && ✓ & E None && *
Consider the following figure.
By rotating the shape around the axis, we get a composite solid. Determine the following measures of this solid in terms of π.
By revolving the figure about the axis, we get a composite solid that consists of two cones attached at their base.
The solid consists of two identical cones with a height of 4 inches and a radius of 3 inches. To determine the volume of a cone, we calculate one-third of the base area times its height. V=1/3π r^2 h Since we have two cones, we must multiply this formula by 2 to get the volume of the composite solid V_(CS).
The volume of the composite solid is 24π cubic inches.
The surface area of the composite solid is twice the surface area of a cone minus their base area. In other words, we only want to calculate the sum of the cone's lateral surfaces.
SA_(CS)=2(π r l)
To calculate the lateral surface, we need the slant height of the cone. As we can see, the slant height is the hypotenuse of a right triangle with legs of 3 and 4 inches. The measurements of such a triangle form a Pythagorean Triple, so the triangle has a hypotenuse of 5 inches.
Let's substitute the radius and slant height into the formula and evaluate the right-hand side.
The surface area of the composite solid is 30π square inches.