Classification of Triangles
Warm-up
Show an equilateral triangle. Ask: How are all the sides? How are all the angles?
Explore
Students sort triangle cards into two piles: first by sides (equilateral, isosceles, scalene), then by angles (acute, right, obtuse). Notice overlap: some triangles are both isosceles AND acute, for example.
Formalize
Draw a 2×3 table on the board:
Classification by Sides: Equilateral (all equal), Isosceles (two equal), Scalene (all different). Classification by Angles: Acute (all < 90°), Right (one = 90°), Obtuse (one > 90°).
Show how a triangle can fit in both categories (e.g., right isosceles). Record examples in each cell.
Practice
Students classify triangles both ways on worksheets and with geoboards. Exit ticket: one triangle to classify by both sides and angles.
Exit ticket
Students classify triangles both ways on worksheets and with geoboards. Exit ticket: one triangle to classify by both sides and angles.
Isosceles (two equal sides) and acute (all angles < 90°). So it's an acute isosceles triangle.
No. In an equilateral triangle, all angles are 60°, so it's always acute.