Naming triangles by their sides (equilateral, isosceles, scalene) and by their angles (acute, right, obtuse), and finding a missing angle, since the three add up to 180°.
For sides, count how many are the same length. For angles, look for the biggest: 90° is right, more is obtuse, all under 90° is acute. A missing angle is 180° minus the other two.
isoscelestwo sides the same length
scaleneno sides the same length
obtuse anglemore than 90°
acute angleless than 90°
Watch for: Calling a triangle acute because it has one acute angle (all three must be). Subtracting only one angle from 180°. Judging the sides by how the triangle is turned, not by the lengths.
At home: Fold a paper corner to make a right angle, then hunt for triangles at home and test their corners with it.
Sample sheet and answer key
Luna's Space Adventure
Grade 5 · Sorting triangles · 5.MG.3
NameDate
Astronaut Luna is exploring a far-off corner of space. Solve each problem to help Luna's mission go smoothly!
Name it by its sides
Equilateral, isosceles or scalene? Lengths are in cm.
Name it by its angles
Acute, right or obtuse?
The missing angle
The three angles of a triangle add up to 180°.
? =°
? =°
? =°
True or false?
Think of a triangle that would prove it wrong.
An equilateral triangle is also isosceles.TrueFalse
A triangle can have two right angles.TrueFalse
Grade 5 · Sheet
Luna's Space Adventure
Grade 5 · Sorting triangles · 5.MG.3
Answer key
Astronaut Luna is exploring a far-off corner of space. Solve each problem to help Luna's mission go smoothly!
Name it by its sides
Equilateral, isosceles or scalene? Lengths are in cm.
isosceles
equilateral
scalene
Name it by its angles
Acute, right or obtuse?
acute
obtuse
right
The missing angle
The three angles of a triangle add up to 180°.
? =63°
? =44°
? =43°
True or false?
Think of a triangle that would prove it wrong.
An equilateral triangle is also isosceles.TrueFalse
A triangle can have two right angles.TrueFalse
Answer key
Something wrong with this sheet? A wrong answer, a hard-to-read picture? Report it
New problems every time, three levels (gentle, core and stretch), themes to pick from, a second page if you want one. Each sheet comes with an answer key and a one-page guide for grown-ups.
