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Obtuse Triangle Calculator

Kaushik RabadiyaCreated by Kaushik RabadiyaLast updated: September 25, 2026

Obtuse triangle instantly calculates results using a1, a2, a3. Use the calculator above for instant answers in your browser.

The Obtuse Triangle Calculator is a specialized geometry tool designed to compute missing side lengths, internal angles, and total surface area when one angle exceeds 90 degrees. Ideal for students, engineers, and architects, this calculator eliminates manual geometric guesswork by instantly processing various combinations of known inputs.

The Mathematics of Obtuse Triangles

An obtuse triangle features one internal angle greater than 90 degrees, while its remaining two angles are strictly acute. The fundamental rule governing all triangles dictates that the sum of all internal angles always equals 180 degrees: angle_one + angle_two + angle_three = 180. To find the area, depending on your known variables, the calculator utilizes standard formulas such as the base-height formula (0.5 * base * height), the side-angle-side sine method (0.5 * a * b * sin(angle)), or Heron's formula using all three known side lengths.

Step-by-Step Calculation Example

Let us walk through finding the area of an obtuse triangle using two side lengths and the included obtuse angle. Suppose you have a triangle where side a = 10 cm, side b = 6 cm, and the included angle is 120 degrees. First, convert the angle to radians or apply the sine formula directly: Area = 0.5 * a * b * sin(angle). Substituting our values, we get Area = 0.5 * 10 * 6 * sin(120°). Since sin(120°) is approximately 0.866, the calculation becomes 30 * 0.866, yielding a final area of approximately 25.98 square centimeters.

Best Practices for Geometric Calculations

Always verify that your angle inputs accurately reflect a valid triangle before calculating. Remember that the longest side of an obtuse triangle is always opposite the obtuse angle. When measuring real-world objects or drafting blueprints, double-check your unit consistency—mixing centimeters and inches will distort your final area and angle computations.

FAQs

What is an obtuse triangle?

An obtuse triangle is a three-sided polygon in which one of the interior angles measures strictly greater than 90 degrees. Because the sum of all angles in any triangle must equal 180 degrees, the other two interior angles must be acute, meaning they measure less than 90 degrees.

How to determine whether the triangle is obtuse?

You can determine if a triangle is obtuse by examining its side lengths or its angles. If you know all three side lengths a, b, and c (where c is the longest side), you can use the Pythagorean inequality theorem: if a squared plus b squared is less than c squared, the triangle is obtuse.

How to calculate the area of an obtuse triangle?

The area can be calculated in several ways depending on available inputs. If you know a base and its corresponding perpendicular height (which may fall outside the main body of the triangle), use the formula 0.5 multiplied by base times height. You can also use trigonometric side-angle-side formulas.

What triangle type has an angle greater than 90 degrees?

An obtuse triangle is specifically defined as having one interior angle greater than 90 degrees. In contrast, right triangles have exactly one 90-degree angle, and acute triangles have all three angles measuring less than 90 degrees.

Formula verified against Mathematical standards (ISO 80000-2) — all calculations use deterministic, standards-based formulas.

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