Carpentry & Building Construction © 2004

Triangles

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Construction workers use triangles in all kinds of jobs. For example, roofers work with triangles when installing trusses. Finish carpenters may work with triangles when installing counters or finishing stairwells.

A triangle is a polygon with three sides and three angles. The sum of the angles in a triangle is always 180 degrees.

There are different kinds of triangles. A right triangle has one 90-degree angle, also called a right angle. An isosceles triangle is a triangle with two equal sides and two equal angles. An equilateral triangle has three equal sides and three equal angles. A scalene triangle has three different side lengths and three different angles.

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To find a missing angle measurement in a triangle, first add the number of degrees in the two known angles. Next, subtract that sum from 180. For example, in the scalene triangle shown here, the two given angles are 35 degrees and 125 degrees. Their sum equals 160 degrees. From 180 degrees, subtract 160 degrees. The remaining angle is 20 degrees.

35º + 125º = 160º

180º - 160º = 20º

Practice Exercise

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Find the missing angle in the right triangle. The given angles are 30 degrees and a right angle, which is 90 degrees.

Problem Exercises

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1.		A Speed Square® is an example of a(n)
			A)	equilateral triangle.
			B)	scalene triangle.
			C)	right triangle.
			D)	regular polygon.
2.		A custom triangle corner shelf has a 50º angle and a 90º angle. What is the remaining angle?
			A)	40º
			B)	45º
			C)	60º
			D)	75º

Use the following figures for the next three problems. The strokes indicate equal sides.

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3.		Find the missing angle in the isosceles triangle.
			A)	90º
			B)	75º
			C)	60º
			D)	45º
4.		The remaining angle in the equilateral triangle is
			A)	30º.
			B)	45º.
			C)	60º.
			D)	90º.
5.		The missing angle on the scalene triangle is
			A)	0º.
			B)	35º.
			C)	60º.
			D)	120º.