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How to Work Out the Sides and Angles of Triangles

Updated on June 11, 2017
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Eugene is a qualified control/instrumentation engineer Bsc (Eng) and has worked as a developer of electronics & software for SCADA systems.

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The Triangle—a 3-Sided Polygon

Polygons are plane (flat) shapes with several straight sides. Examples are squares, triangles, hexagons, and pentagons. The name originates from the Greek "polús" meaning "many" and "gōnía" meaning "corner" or "angle." So polygon means "many corners."

A triangle is the simplest polygon, having 3 sides.

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Triangles Have Angles

Triangles have 3 corners and the angle between the sides can be anything from greater than 0 up to 180 degrees. The angles can't be 0 or 180 degrees because then the triangles would become straight lines.

Degrees can be written using the symbol º so 45º means 45 degrees.

Angles of a triangle range from  0 to   180 degrees
Angles of a triangle range from 0 to 180 degrees | Source
Triangles classed by side and angles
Triangles classed by side and angles | Source

All the Angles Add Up To 180 degrees

For every triangle in the known Universe, all the angles add up to 180 degrees.

No matter what the shape or size of a triangle, the sum of the 3 angles is 180 degrees
No matter what the shape or size of a triangle, the sum of the 3 angles is 180 degrees | Source

Similar Triangles

Similar triangles have exactly the same angles but different length sides.
The sides are in the same ratio.

Similar triangles
Similar triangles

The Greek Alphabet

In science, mathematics, and engineering many of the 24 characters of the Greek alphabet are "borrowed" for use in diagrams and for describing certain quantities. For example, the characters θ (theta) and φ (phi) are often used for representing angles. You may have seen the character μ (mu) to represent micro as in micrograms μg or micrometers μm. The capital letter Ω (omega) is the symbol for ohms in electrical engineering. And of course π (pi) is the ratio of the circumference to the diameter of a circle

The Greek alphabet
The Greek alphabet | Source

Using Trigonometry For Solving Triangles

Trigonometry is a branch of mathematics which deals with the relationship between the lengths of the sides of a triangle and its angles. If we don't know all the lengths of the sides of a triangle and/or don't know all the angles, there are several equations which can be used to work them out. Working out sides and angles is known as solving triangles.

Triangles Can be Solved Using:

  • Pythagoras's Theorem
  • Sine Rule
  • Cosine Rule
  • Angles all add up to 180 degrees

Pythagoras's Theorem (The Pythagorean Theorem)

Pythagoras's Theorem (also known as the Pythagorean theorem) states that for a right angled triangle

"The square on the hypotenuse equals the sum of the squares on the other two sides."

So if you know the lengths of 2 sides, all you have to do is square the two lengths, add the results and then take the square root of the sum to get the length of the hypotenuse.

Pythagoras's Theorem
Pythagoras's Theorem | Source

Sine and Cosine

A right-angled triangle has one angle measuring 90 degrees. The side opposite this angle is known as the hypotenuse and it is the longest side. Sine and cosine are trigonometric functions of an angle and are the ratios of the lengths of the other two sides to the hypotenuse of a right-angled triangle.

In the diagram below, one of the angles is represented by the Greek letter θ.

The side a is known as the "opposite" side and side b is the "adjacent" side to the angle θ.

sine θ = length of opposite side / length of hypotenuse

cosine θ = length of adjacent side / length of hypotenuse

Sine and cosine apply to an angle, not necessarily an angle in a triangle, so it's possible to just have two lines meeting at a point and to evaluate sine or cos for that angle. However sine and cos are derived from the sides of an imaginary right angled triangle superimposed on the lines. In the second diagram below, you can visualise a right angled triangle superimposed on the purple triangle, from which the opposite and adjacent sides and hypotenuse can be determined.

Over the range 0 to 90 degrees, sine ranges from 0 to 1 and cos ranges from 1 to 0

Remember sine and cosine only depend on the angle, not the size of the triangle. So if the length a changes in the diagram below when the triangle changes in size, the hypotenuse c also changes in size, but the ratio of a to c remains constant.

Sine and cosine are sometimes abbreviated to sin and cos

Sine and cosine
Sine and cosine

The Sine Rule

The ratio of the length of a side of a triangle to the sine of the angle opposite it is constant for all 3 sides and angles.

So in the diagram below

a / sine A = b / sine B = c / sine C

Now you can check the sine of an angle using a scientific calculator or look it up online. In the old days before scientific calculators, we had to look up the value of the sine or cos of an angle in a book of tables.

The opposite or reverse function of sine is arcsine, sometimes written as sin-1. When you check the arcsine of a value, you're working out the angle which produced that value when the sine function was operated on it.

So sin (30º) = 0.5 and sin-1(0.5) = 30º

The Sine Rule can be used:

If the length of one side is known and the magnitude of the angle opposite it, then if any of the other remaining angles or sides are known, all the angles and sides can be worked out.

Sine rule
Sine rule | Source
Sine rule example
Sine rule example | Source

The Cosine Rule

For a triangle with sides a, b and c, then if a and b are known and C is the included angle (the angle between the sides), c can be worked out from:

c2 = a2 + b2 - 2abCos C

The Cosine Rule can be used if:

(a) You know the lengths of two sides of a triangle and the included angle. You can then work out the length of the remaining side using the Cosine Rule.

(b) You know all the lengths of the sides but none of the angles.

Then rearranging the cosine rule equation:

C = Arccos ((a2 + b2 - c2) / 2ab)

The other angles can be worked out similarly.


Cosine rule
Cosine rule | Source
Cosine rule example
Cosine rule example | Source

Area of a Triangle

The area of a triangle can be determined by multiplying half the length of its base by the perpendicular height. Perpendicular means at right angles. But which side is the base? Well, you can use any of the three sides. On a drawing, you can work out the area by drawing a perpendicular line from one side to the opposite corner using a set square, T-square or protractor. (or a carpenter's square if you're constructing something). Then measure the length of the line and use the formula 1/2ah to get the area.

Alternative Method of Working Out Area

The simple method above requires that you actually measure the height of a triangle. If you know the length of 2 of the sides and the included angle, you can work out the area analytically.

Area of a triangle equals half the base length multiplied by the perpendicular height
Area of a triangle equals half the base length multiplied by the perpendicular height | Source
Area = 1/2 the product of the sides multiplied by the sine of the included angle
Area = 1/2 the product of the sides multiplied by the sine of the included angle | Source

Triangles Are Strong!

A triangle is the most basic polygon and can't be pushed out of shape easily unlike a square. If you look closely enough, triangles are used everywhere in machines and structures because the shape is so strong.

Truss bridge
Truss bridge | Source
A roof truss
A roof truss | Source
Spoked wheels
Spoked wheels | Source
Source
Source
Can you spot the triangles?
Can you spot the triangles? | Source

Using a Triangle Calculator

There are lots of online triangle calculators on the web which make things easy (or take the fun out of doing the calculations!). These calculators allow you to enter angles and side lengths and they will work out all the remaining sides and angles. Here's one here:

http://www.trianglecalculator.net

© 2016 Eugene Brennan

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    • ronbergeron profile image

      Ron Bergeron 11 months ago from Massachusetts, US

      I've always found the math behind triangles to be interesting. I'm glad that you ended the hub with some examples of triangles in every day use. Showing a practical use for the information presented makes it more interesting and demonstrates a purpose for learning about it.

    • eugbug profile image
      Author

      Eugene Brennan 11 months ago from Ireland

      Thanks Ron, triangles are great, they crop up everywhere in structures, machines, and the ligaments of the human body can be thought of as ties, forming one side of a triangle.

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