Draw a triangle on tracing paper. Fold the altitude from each vertex.
10.2
Activity
Altitude Attributes
Triangle is graphed.
Find the slope of each side of the triangle.
Find the slope of each altitude of the triangle.
Sketch the altitudes. Label the point of intersection, .
Write equations for all 3 altitudes.
Use the equations to find the coordinates of , and verify algebraically that the altitudes all intersect at .
10.3
Activity
Percolating on Perpendicular Bisectors
Triangle is graphed.
Find the midpoint of each side of the triangle.
Sketch the perpendicular bisectors, using an index card to help draw 90-degree angles. Label the intersection point as .
Write equations for all 3 perpendicular bisectors.
Use the equations to find the coordinates of , and verify algebraically that the perpendicular bisectors all intersect at .
10.4
Activity
Tiling the (Coordinate) Plane
A tessellation covers the entire plane with shapes that do not overlap or leave gaps.
Tile the plane with congruent rectangles:
Draw the rectangles on your grid.
Write the equations for lines that outline 1 rectangle.
Tile the plane with congruent right triangles:
Draw the right triangles on your grid.
Write the equations for lines that outline 1 right triangle.
Tile the plane with any other shapes:
Draw the shapes on your grid.
Write the equations for lines that outline 1 of the shapes.
Student Lesson Summary
The three perpendicular bisectors of a triangle always intersect in one point. We can use coordinate geometry to show that the altitudes of a triangle intersect in one point, too. The three altitudes of triangle are shown here. They appear to intersect at the point . By finding their equations, we can prove this is true.
The slopes of sides and are 0, , and 2. The altitude from is the vertical line . The slope of the altitude from is . Since the altitude goes through its equation is . The slope of the altitude from is . Following this slope over to the -axis we can see that the -intercept is 8. So the equation for this altitude is .
We can now verify that lies on all three altitudes by showing that the point satisfies the three equations. By substitution, we see that each equation is true when and .