Calculating the Diagonal of a Triangle

Published
<h3>Calculating the Diagonal of a Triangle</h3>

If your teacher has asked you to calculate the diagonal of a triangle, she’s already given you some valuable information. That phrasing tells you that you’re dealing with a right triangle, where two sides are perpendicular to each other (or to say it another way, they form a right triangle) and only one side is left to be “diagonal” to the others. That diagonal is called the hypotenuse, and you can find its length using the Pythagorean Theorem.

TL;DR (Too Long; Didn’t Read)

To find the length of the diagonal (or hypotenuse) of a right triangle, substitute the lengths of the two perpendicular sides into the formula ​_a2_​ + ​_b2_​ = ​_c2_​, where ​_a_​ and ​_b_​ are the lengths of the perpendicular sides and ​_c_​ is the length of the hypotenuse. Then solve for ​_c_​.



Pythagoras’ Theorem

The Pythagorean Theorem – sometimes also called Pythagoras’ Theorem, after the Greek philosopher and mathematician who discovered it – states that if ​a​ and ​b​ are the lengths of the perpendicular sides of a right triangle and ​c​ is the length of the hypotenuse, then:

(a^2 + b^2 = c^2)

In real-world terms, this means that if you know the length of any two sides of a right triangle, you can use that information to find out the length of the missing side. Note that this only works for right triangles.

Solving for the Hypotenuse

Assuming you know the lengths of the two non-diagonal sides of the triangle, you can substitute that information into the Pythagorean Theorem and then solve for ​c.​



1. Substitute Values for a and b

Substitute the known values of ​a​ and ​b​ – the two perpendicular sides of the right triangle – into the Pythagorean Theorem. So if the two perpendicular sides of the triangle measure 3 and 4 units respectively, you’d have:

(3^2 + 4^2 = c^2)

2. Simplify the Equation

Work the exponents (when possible – in this case you can) and simplify like terms. This gives you:

See also  Understanding the Difference Between a Jet and a Plane


(9 + 16 = c^2)

Followed by:

(c^2 = 25)

3. Take the Square Root of Both Sides

Take the square root of both sides, the final step in solving for ​c​. This gives you:

(c = sqrt{25}= 5)

So the length of the diagonal, or hypotenuse, of this triangle is 5 units.

Dave Pennells

By Dave Pennells

Dave Pennells, MS, has contributed his expertise as a career consultant and training specialist across various fields for over 15 years. At City University of Seattle, he offers personal career counseling and conducts workshops focused on practical job search techniques, resume creation, and interview skills. With a Master of Science in Counseling, Pennells specializes in career consulting, conducting career assessments, guiding career transitions, and providing outplacement services. Her professional experience spans multiple sectors, including banking, retail, airlines, non-profit organizations, and the aerospace industry. Additionally, since 2001, he has been actively involved with the Career Development Association of Australia.