Students must be confident in identifying angles before they can classify triangles. Review acute, right and obtuse angles. Ask a volunteer to walk to the classroom door and open it into an acute angle. Remind students that an acute angle is less than 90 degrees. Have another student open the door to form a right angle -- exactly 90 degrees. Ask why the door cannot be opened into an obtuse angle -- greater than 90 degrees. Instruct all students to form acute angles with their arms. Follow the same procedure for right and obtuse angles. Solicit volunteers to draw various angle types on the board.
Place students into groups of two or three. Provide straws, scissors, glue, markers, protractors and a sheet of poster board. Tell students to create six triangles out of straws, glue them on the poster board and label them with a marker. The top half of the poster includes triangles classified by sides: isosceles -- two sides equal; equilateral -- all sides equal; and scalene -- no sides equal. The bottom half of the poster includes triangles classified by angles: acute -- one angle less than 90 degrees; obtuse -- one angle greater than 90 degrees; and right -- one angle exactly 90 degrees.
For demonstration purposes, use an online geometry board and an overhead projection to create a triangle on a geoboard. Give each student a geoboard and a bag of rubber bands. Tell students to use the rubber bands to shape right triangles on their geoboards. Ask students to face the boards toward you when they finish, so you can check the resulting triangles. Continue this process, including triangles classified both by sides and by angles.
Download a black-line, quilt block pattern from an online source. Make sure the design includes various types of triangles. Distribute one to each student. Instruct students to color all the acute triangles red, the right triangles blue and the obtuse triangles yellow. Challenge students of higher ability levels to create their own quilt patterns.
Arrange students in groups of three or four. Give each student a bag of pipe cleaners and give each group a deck of cards labeled “acute,” “right,” “obtuse,” “scalene,” equilateral” and “isosceles.” Students place the cards face down. They turn over the top card and use the pipe cleaners to form the given triangle. The first student with the correct shape earns 10 points. Continue the challenge with the other cards. The winner is the student with the most points.