Geometry Distance of Triangles using a Protractor
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Eileen Lally A. Philip Randolph Magnet School
7316 South Hoyne Avenue
Chicago IL 60636
Students in the 7th grade are to learn how to use the protractor to measure
angles, and use this ability to solve a problem involving distance.
Protractors for each student
Rulers and meter sticks
Straws and clay
1. Identify the vocabulary: ray, angle, vertex, unit of measure,
2. Demonstrate or review how to use the protractor.
3. Draw a 60cm line labeled AB on the board. Instruct students to draw a 6cm
line labeled ab on paper.
4. At point A/a make a 35o and at point B/b make a 60o. Make sure that the rays
are extended until they cross. Label that point C.
5. Compare the triangles (the one on the board and the one on paper).
These triangles are similar.
6. Present the question: What is the distance of the line segment AC without
leaving your desk?
7. Set up the ratio: line AB over line ab = X (AC) over line ac.
8. Now measure the line AC and compare the result with the calculated answer.
Use the formula: Actual measurement minus Calculated measurement
divided by Actual to obtain the margin of error.
1. Upon a large table, mark two points A and B and determine the distance
(AB) between them.
2. Use the protractor to determine the measure of angle BAC. Likewise,
determine the measure of angle ABC.
3. We now attempt to determine X, the distance (AC) from the point A to the
Point C. Make a model (as described in Part 1) keeping the angles found but
reduce the size of line segment ab. Set up the ratio: Line AB over line ab
= X (AC) over line ac.
4. To verify, measure the distance from point A to point C. Compare with the
calculated answer. Use the formula: Actual measurement minus Calculated
measurement divided by Actual to obtain the margin of error.
A similar problem like part two can be used as a performance assessment.
The students are to answer the following question. What is the length of
Knowing two angles and the distance between them, you can find the distance
of the point that completes the triangle. This can be done by making a
smaller model to help calculate the answer.