1.7: Special Triangle Ratios
While working in your Industrial Arts class one day, your Instructor asks you to use your 454590 triangle to make a scale drawing. Unfortunately, you have two differently shaped triangles to use at your drafting table, and there aren't labels to tell you which triangle is the correct one to use.
You turn the triangles over and over in your hands, trying to figure out what to do, when you spot the ruler at your desk. Taking one of the triangles, you measure two of its sides. You determine that the first side is 7 inches long, and the second side is just a little under 9.9 inches. Can you determine if this is the correct triangle for your work?
At the completion of this Concept, you'll be able to verify if this is the correct triangle to use.
Watch This
James Sousa Solving Special Right Triangles
Guidance
Special right triangles are the basis of trigonometry. The angles \begin{align*}30^\circ, \ 45^\circ, \ 60^\circ\end{align*}
First, let’s compare the two ratios, so that we can better distinguish the difference between the two. For a \begin{align*}454590\end{align*}
Example A
Determine if the set of lengths below represents a special right triangle. If so, which one?
\begin{align*}8\sqrt{3}:24:16\sqrt{3}\end{align*}
Solution: Yes, this is a \begin{align*}306090\end{align*}
Example B
Determine if the set of lengths below represents a special right triangle. If so, which one?
\begin{align*}\sqrt{5}:\sqrt{5}:\sqrt{10}\end{align*}
Solution: Yes, this is a \begin{align*}454590\end{align*}
Example C
Determine if the set of lengths below represents a special right triangle. If so, which one?
\begin{align*}6\sqrt{7}:6\sqrt{21}:12\end{align*}
Solution: No, this is not a special right triangle. The hypotenuse should be \begin{align*}12\sqrt{7}\end{align*}
Guided Practice
1. Determine if the set of lengths below represents a special right triangle. If so, which one?
\begin{align*}3\sqrt{2}:3\sqrt{2}:6\end{align*}
2. Determine if the set of lengths below represents a special right triangle. If so, which one?
\begin{align*}4:2:2\sqrt{3}\end{align*}
3. Determine if the set of lengths below represents a special right triangle. If so, which one?
\begin{align*}13:84:85\end{align*}
Solutions:
1. The sides are the same length. This means that if the triangle is one of the special triangles at all, it must be a 454590 triangle. To test this, we take either of the sides that are equal and multiply it by \begin{align*}\sqrt{2}\end{align*}
\begin{align*}3\sqrt{2} \times \sqrt{2} = 3 \times \sqrt{4} = 3 \times 2 = 6\end{align*}
Yes, this triangle is a special triangle. It is a 454590 triangle.
2. It can immediately be seen that the second side is one half the length of the first side. This means that if it is a special triangle, it must be a 306090 triangle. To see if it is indeed such a triangle, look at the relationship between the shorter side and the final side. The final side is \begin{align*}\sqrt{3}\end{align*}
3. It can be seen immediately that the lengths of sides given aren't a special triangle, since 84 is so close to 85. Therefore it can't be a 454590 triangle, which would require \begin{align*}84\sqrt{2}\end{align*}
Concept Problem Solution
Since you know the ratios of lengths of sides for special triangles, you can test to see if the triangle in your hand is the correct one by testing the relationship:
hypotenuse = \begin{align*}\sqrt{2}x\end{align*}
where "x" is the length of the shorter sides. If you test this relationship with the triangle you are holding:
hypotenuse = \begin{align*}7\sqrt{2} = 9.87\end{align*}
Yes, you are holding the correct triangle.
Explore More
For each of the set of lengths below, determine whether or not they represent a special right triangle. If so, which one?

\begin{align*}2:2:2\sqrt{2}\end{align*}
2:2:22√ 
\begin{align*}3:3:6\end{align*}
3:3:6  \begin{align*}3:3\sqrt{3}:6\end{align*}
 \begin{align*}4\sqrt{2}:4\sqrt{2}:8\sqrt{2}\end{align*}
 \begin{align*}5\sqrt{2}:5\sqrt{2}:10\end{align*}
 \begin{align*}7:7\sqrt{2}:14\end{align*}
 \begin{align*}6\sqrt{5}:18\sqrt{5}:12\sqrt{5}\end{align*}
 \begin{align*}4\sqrt{6}:12\sqrt{2}:8\sqrt{6}\end{align*}
 \begin{align*}8\sqrt{15}:24\sqrt{5}:16\end{align*}
 \begin{align*}7\sqrt{6}:7\sqrt{6}:14\sqrt{3}\end{align*}
 \begin{align*}5\sqrt{7}:5\sqrt{14}:5\sqrt{7}\end{align*}
 \begin{align*}9\sqrt{6}:27\sqrt{2}:18\sqrt{6}\end{align*}
 Explain why if you cut any square in half along its diagonal you will create two 454590 triangles.
 Explain how to create two 306090 triangles from an equilateral triangle.
 Could a special right triangle ever have all three sides with integer lengths?
Image Attributions
Description
Learning Objectives
Here you'll learn how to determine if a given triangle is a 306090 or a 454590 triangle by examining the relationship between the lengths of the sides.
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Date Created:
Sep 26, 2012Last Modified:
Feb 26, 2015Vocabulary
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