Look at the biggest angle
The largest angle decides acute, right, or obtuse.
Why it works
One square corner → right.
Geometry · Triangles & polygons · 2.2
Types of triangles.
Triangles are classified by their angles — acute (all < 90°), right (one 90°), or obtuse (one > 90°) — and by their sides — equilateral (all equal), isosceles (two equal), or scalene (all different).
See it work.
Watch the method
Types of triangles
Right has a 90° angle; obtuse has one over 90°; acute has all under 90°.
1
Area of a rectangle
Area is length times width.
2
Multiply
Multiply the two dimensions.
The idea underneath.
From easy to tricky.
A triangle has angles 90°, 42°, and 48°. By its angles, it is…
1
Answer
Right
A triangle has angles 63°, 67°, and 50°. By its angles, it is…
1
Answer
Acute
A triangle has angles 77°, 51°, and 52°. By its angles, it is…
1
Answer
Acute
Where students slip.
Trap
Calling any large triangle obtuse.
Fix
Obtuse means an angle OVER 90°, not just big.
Trap
Mixing angle and side names.
Fix
Right/acute/obtuse are by angle; equilateral/isosceles by sides.
Test yourself.
The Test
12 problems across three tiers. Auto-graded on submit. Hints and a full walkthrough on every question. Your work is saved on this device — sign in to keep it across devices.
0/12Answered
How to answerType just the value. Formatting won't trip you up — we ignore spaces, capital letters, commas, and subscript style, so
t₁ = 5, d = 3 and t1=5,d=3 both count. For a list of numbers, separate them with commas (e.g. 3, 10, 17).Core
Core (4)
C1
A triangle has angles 116°, 36°, and 28°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
C2
A triangle has angles 63°, 61°, and 56°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
C3
A triangle has angles 90°, 64°, and 26°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
C4
A triangle has angles 90°, 38°, and 52°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
Apply
Apply (4)
A1
A triangle has angles 129°, 23°, and 28°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
A2
A triangle has angles 78°, 52°, and 50°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
A3
A triangle has angles 140°, 20°, and 20°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
A4
A triangle has angles 90°, 58°, and 32°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
Extension
Extension (4)
E1
A triangle has angles 90°, 38°, and 52°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
E2
A triangle has angles 50°, 72°, and 58°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
E3
A triangle has angles 74°, 52°, and 54°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.
E4
A triangle has angles 110°, 46°, and 24°. By its angles, it is…
Need a hint?
- Find the largest angle.
- Compare it to 90°.