4.6.C
The Standard
apply knowledge of right angles to identify acute, right, and obtuse triangles
Texas Essential Knowledge and Skills for Mathematics
Teacher's field guide
What This Standard Means
What Students Need to Do
- Students use a right angle as a benchmark when examining the angles in a triangle. They decide whether all angles are smaller, one is square, or one is larger.
What Mastery Looks Like
- Students correctly classify triangles even when the figures are rotated or drawn with unequal sides. They compare each angle with a right angle and explain which angle evidence supports the name.
Common Misconceptions
- Students may classify a triangle by how it is turned or by the length of its sides. They may also call a triangle acute after checking only one angle, instead of checking all three.
How to Assess It
- Give students drawings of three triangles, one acute, one right, and one obtuse, in different orientations. Ask them to classify each and mark the angle evidence.
Lesson moves
Ways to Teach It
Fold a paper corner to make a right-angle tester, then use it to check and label every angle on triangle cards.
Ask students to write: Which angles must you check before naming a triangle, and why?
Play a triangle card sort where partners place each card under acute, right, or obtuse and challenge one placement with evidence.
Show photos of roof trusses, road signs, and bridge supports, then have students identify and justify each visible triangle type.
Keep exploring
Related Standards
- 6.8.A
extend previous knowledge of triangles and their properties to include the sum of angles of a triangle, the relationship between the lengths of sides and measur...
- 4.7
The student applies mathematical process standards to solve problems involving angles less than or equal to 180 degrees
- 8.8.D
use informal arguments to establish facts about the angle sum and exterior angle of triangles, the angles created when parallel lines are cut by a transversal, ...
- 7.11.C
write and solve equations using geometry concepts, including the sum of the angles in a triangle, and angle relationships
Turn this exact standard into a lesson
Grade, subject, topic, and the complete standard are prefilled. Create one free, no account needed.