Maryland Math 5.GR.B.3.a

Math5th GradeGeometric Reasoning and Measurement

The Standard

Classify triangles based on properties (e.g., angle measure, side lengths).

Maryland College and Career Ready Standards for Mathematics (2025) · CLASSIFY TWO-DIMENSIONAL FIGURES BASED ON PROPERTIES.

Teacher's field guide

What Students Need to Do: Students classify triangles by their side lengths and angle measures. They use side marks, rulers, and protractors to support their answers. They give both a side label and an angle label.
What Mastery Looks Like: A student can read or measure a triangle, then name its side and angle types. They can explain why a triangle with two equal sides and a right angle is both isosceles and right.
Common Misconceptions: Students may think a triangle can have only one label. They may also judge angles by how a triangle is tilted instead of measuring them.
How to Assess It: Show a triangle with two sides labeled 4 cm and a marked right angle between them. Ask, "Classify it by sides and angles. What evidence supports each name?"

Lesson moves

Build triangles with 4 cm, 4 cm, and 6 cm straws, then measure their angles and label both types.
Show a right triangle with two equal legs and ask, "Can a triangle be both right and isosceles? Explain."
Give partners triangle cards with marked angles and side lengths, then have them sort each card by both properties.
Trace a triangular shelf bracket on paper, measure its sides and angles, and label both types.

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