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Acute Triangle – Definition, Examples | EDU.COMEDU.COMResourcesBlogGuidePodcastPlanBackHomesvg]:size-3.5">Math Glossarysvg]:size-3.5">Acute TriangleAcute Triangle – Definition, ExamplesTable of ContentsAcute Triangle Definition of Acute Triangle

An acute triangle is a triangle in which all three internal angles measure less than 90 degrees. This special type of triangle is also known as an acute-angled triangle. While the sides of acute triangles may vary in length, the defining characteristic is that none of the interior angles can be 90 degrees or larger.

Acute triangles can be further classified into three types based on their side lengths and angles. Equilateral triangles have three equal sides and three equal angles of 60 degrees each, making them always acute. Acute isosceles triangles have two equal sides and two equal angles (both less than 90 degrees). Acute scalene triangles have no equal sides and no equal angles, with all angles measuring less than 90 degrees.

Examples of Acute Triangle Example 1: Finding a Missing Angle in an Acute Triangle Problem:

Find the missing angle in the given figure and identify the type of triangle.

triangle

Step-by-step solution:

Step 1, recall that the sum of all angles in a triangle is always 180°. We can use this to find the missing angle.

Step 2, look at the two known angles in the triangle. One angle is 70° and another is 30°.

Step 3, use the angle sum property to find the unknown angle: ∠A = 180° - 70° - 30° = 80°.

Step 4, check if this is an acute triangle by making sure all three angles are less than 90°. We have 70°, 30°, and 80°, which are all less than 90°, so this is an acute triangle.

Step 5, decide what type of acute triangle it is. Since all three angles of the triangle are different (70°, 30°, and 80°), it is an acute scalene triangle.

Example 2: Finding the Length of a Side in an Acute Isosceles Triangle Problem:

In an acute isosceles triangle ABC, side AB = 6 cm and ∠B = ∠C. If the perimeter of this triangle is 16 cm, then find the length of the side BC.

triangle

Step-by-step solution:

Step 1, understand what makes this an isosceles triangle. In the triangle ABC, we know that ∠B = ∠C, which means the sides opposite to these angles must be equal.

Step 2, identify the equal sides. The sides opposite to angles B and C are AC and AB respectively. Since ∠B = ∠C, we know that AB = AC = 6 cm.

Step 3, use the perimeter formula to find the unknown side. The perimeter of a triangle is the sum of all three sides: AB + AC + BC = 16 cm.

Step 4, substitute what we know: 6 cm + 6 cm + BC = 16 cm.

Step 5, solve for BC: BC = 16 cm - 6 cm - 6 cm = 4 cm.

Example 3: Calculating the Area of an Acute Triangle Problem:

Find the area of the acute triangle whose base is 5 cm long and height is 6 cm.

triangle

Step-by-step solution:

Step 1, recall the formula for the area of a triangle: Area = (12\frac{1}{2}21​) × base × height.

Step 2, identify the given values. We know that the base (b) is 5 cm and the height (h) is 6 cm.

Step 3, substitute these values into the area formula: Area = (12\frac{1}{2}21​) × 5 cm × 6 cm.

Step 4, calculate the area: Area = (12\frac{1}{2}21​) × 30 cm² = 15 cm².

Comments(6)PPRSpecialistVinceNovember 4, 2025I've used this acute triangle def to teach my students. It's super clear & the examples really helped them grasp the concept!

MCMs. CarterSeptember 17, 2025I’ve used the acute triangle definition and examples from this page to help my kids with their geometry homework. It’s clear and easy to understand, plus the practical examples were super helpful!

NNatureLover87September 10, 2025I used the acute triangle definition and examples to help my kids with their homework, and it made things so much clearer! The diagrams and step-by-step examples were a huge help.

MCMs. CarterAugust 27, 2025I used this page to help my kids understand acute triangles, and it’s so clear! The examples made it easy for them to see the difference between types of triangles. Great resource!

MCMs. CarterAugust 20, 2025I’ve been using this page to help my kids understand acute triangles better, and it’s been a game-changer! The examples are easy to follow, and they loved exploring different triangle types. Highly recommend!

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