4.43: 30-60-90 Right Triangles
- Page ID
- 4984
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Hypotenuse equals twice the smallest leg, while the larger leg is \(\sqrt{3}\) times the smallest.
One of the two special right triangles is called a 30-60-90 triangle, after its three angles.
30-60-90 Theorem: If a triangle has angle measures \(30^{\circ}\), \(60^{\circ}\) and \(90^{\circ}\), then the sides are in the ratio \(x:x\sqrt{3}:2x\).
The shorter leg is always x, the longer leg is always \(x\sqrt{3}\), and the hypotenuse is always \(2x\). If you ever forget these theorems, you can still use the Pythagorean Theorem.
What if you were given a 30-60-90 right triangle and the length of one of its side? How could you figure out the lengths of its other sides?
Example \(\PageIndex{1}\)
Find the value of \(x\) and \(y\).
Solution
We are given the longer leg.
\(\begin{aligned} &x\sqrt{3} =12 \\ &x=\dfrac{12}{\sqrt{3}}\cdot \dfrac{\sqrt{3}}{\sqrt{3}}=\dfrac{12\sqrt{3}}{3}=4\sqrt{3} \\ &\text{The hypotenuse is} \\ &y=2(4\sqrt{3})=8\sqrt{3}\end{aligned}\)
Example \(\PageIndex{2}\)
Find the value of \(x\) and \(y\).
Solution
We are given the hypotenuse.
\(\begin{aligned}&2x=16 \\ &x=8 \\ &\text{The longer leg is} \\ &y=8\cdot \sqrt{3}=8\sqrt{3}\end{aligned}\)
Example \(\PageIndex{3}\)
Find the length of the missing sides.
Solution
We are given the shorter leg. If \(x=5\), then the longer leg, \(b=5\sqrt{3}\), and the hypotenuse, \(c=2(5)=10\).
Example \(\PageIndex{4}\)
Find the length of the missing sides.
Solution
We are given the hypotenuse. \(2x=20\), so the shorter leg, \(f=\dfrac{20}{2}=10\), and the longer leg, \(g=10\sqrt{3}\).
Example \(\PageIndex{5}\)
A rectangle has sides 4 and \(4\sqrt{3}\). What is the length of the diagonal?
Solution
The two lengths are \(x\), \(x\sqrt{3}\), so the diagonal would be \(2x\), or \(2(4)=8\).
If you did not recognize this is a 30-60-90 triangle, you can use the Pythagorean Theorem too.
\(\begin{aligned} 4^2+(4\sqrt{3})^2&=d^2 \\ 16+48&=d^2 \\ d&=\sqrt{64}=8\end{aligned}\)
Review
- In a 30-60-90 triangle, if the shorter leg is 5, then the longer leg is __________ and the hypotenuse is ___________.
- In a 30-60-90 triangle, if the shorter leg is x, then the longer leg is __________ and the hypotenuse is ___________.
- A rectangle has sides of length 6 and \(6\sqrt{3}\). What is the length of the diagonal?
- Two (opposite) sides of a rectangle are 10 and the diagonal is 20. What is the length of the other two sides?
For questions 5-12, find the lengths of the missing sides. Simplify all radicals.
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Review (Answers)
To see the Review answers, open this PDF file and look for section 8.6.
Resources
Vocabulary
| Term | Definition |
|---|---|
| 30-60-90 Theorem | If a triangle has angle measures of 30, 60, and 90 degrees, then the sides are in the ratio \(x : x \sqrt{3} : 2x\) |
| 30-60-90 Triangle | A 30-60-90 triangle is a special right triangle with angles of \(30^{\circ}\), \(60^{\circ}\), and \(90^{\circ}\). |
| Hypotenuse | The hypotenuse of a right triangle is the longest side of the right triangle. It is across from the right angle. |
| Legs of a Right Triangle | The legs of a right triangle are the two shorter sides of the right triangle. Legs are adjacent to the right angle. |
| Pythagorean Theorem | The Pythagorean Theorem is a mathematical relationship between the sides of a right triangle, given by \(a^2+b^2=c^2\), where a and b are legs of the triangle and c is the hypotenuse of the triangle. |
| Radical | The \(\sqrt\), or square root, sign. |
Additional Resources
Interactive Element
Video: Solving Special Right Triangles
Activities: 30-60-90 Right Triangles Discussion Questions
Study Aids: Special Right Triangles Study Guide
Practice: 30-60-90 Right Triangles
Real World: Fighting the War on Drugs Using Geometry and Special Triangles