Grade 4

Share the Headline Story with the class. Students may work with Cuisenaire© Rods (light green and yellow) for the two given sides of the triangle and try to figure out what rods could make the third side. For an added challenge, and surprise, ask them to solve the same problem if the two given sides are 3 cm and 10 cm.

Two sides of a triangle are 3 cm and 5 cm. The third side is a whole number of centimeters long. What are the possible lengths of the third side?

- The third side must be at least 3 cm long, because if it were only 2 cm, we couldn’t make a triangle.
- It can’t be over 7 cm long, because 8 cm wouldn’t make a triangle. So it must be 3, 4, 5, 6, or 7 cm long!
- Amazingly, if the first two sides are 3 cm and 10 cm, we still have only 5 possible sizes for the third side: 8, 9, 10, 11, and 12 cm.

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