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Relationships of Sides in 30-60-90 Right Triangles

Hypotenuse equals twice the smallest leg, while the larger leg is sqrt(3) times the smallest.

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Relationships of Sides in 30-60-90 Right Triangles

You are working on a project in your Industrial Arts class. You have been instructed to design a building. You have a plastic triangular piece that helps you with straight edges and designs. This triangle has interior angles of , , and . Laying the triangle flat on the paper, you decide to use it to help you with the right angle at the building's base.

You trace out the bottom and left edge of the triangle on the paper to serve as the side and bottom of the structure. Looking at the length of the base of the building, you see that it is 7 inches long. Can you determine what the height of the building is from this information?

Read on, and at the conclusion of this Concept you'll be able to do just that.

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James Sousa Examples: Solve a 30-60 Right Triangle

Guidance

refers to each of the angles in this special right triangle. To understand the ratios of the sides, start with an equilateral triangle with an altitude drawn from one vertex.

Recall from geometry that an altitude, , cuts the opposite side directly in half. So we know that one side, the hypotenuse, is and the shortest leg is . Also, recall that the altitude is a perpendicular and angle bisector, which is why the angle at the top is split in half. To find the length of the longer leg, use the Pythagorean Theorem:

From this we can conclude that the length of the longer leg is the length of the short leg multiplied by or . Just like the isosceles right triangle, we now only need one side in order to determine the other two in a triangle. The ratio of the three sides is written , where is the shortest leg, is the longer leg and is the hypotenuse.

Notice, that the shortest side is always opposite the smallest angle and the longest side is always opposite .

Example A

Find the lengths of the two missing sides in the triangle.

Solution: Determine which side in the ratio is given and solve for the other two.

is the longer leg because it is opposite the . So, in the ratio, , therefore and . The short leg is 4 and the hypotenuse is 8.

Example B

Find the lengths of the two missing sides in the triangle.

Solution: Determine which side in the ratio is given and solve for the other two.

17 is the hypotenuse because it is opposite the right angle. In the ratio, and so the short leg is and the long leg is .

Example C

Find the lengths of the two missing sides in the triangle.

Solution: Determine which side in the ratio is given and solve for the other two.

15 is the long leg because it is opposite the . Even though 15 does not have a radical after it, we can still set it equal to .

Multiplying by 2, we get the hypotenuse length, which is .

Guided Practice

1. Find the lengths of the two missing sides in the triangle.

2. Find the lengths of the two missing sides in the 30-60-90 triangle below.

3. Find the angles in the triangle.

Solutions:

1. Since the given side has a length of 8 and is the side which is opposite the right angle, we know that this is the "2x" side of the triangle. Therefore, the short side of the triangle is and the third side of the triangle is .

2. We can see that the shortest side of the triangle has a length of 3 while the longest side has a length of 6 and the other side has a length of . This means that the angle is opposite the side with length , the angle is opposite the side with length , and the angle is opposite the side with length .

3. The length of the given side is , and is opposite the angle. This means that the side opposite the angle is , and the length of the side opposite the angle is

Concept Problem Solution

Since you know the ratios of sides of a 30-60-90 triangle, you know that since the bottom side has a length of cm, the left side must have a length of cm.

Explore More

1. In a 30-60-90 triangle, if the shorter leg is 8, then the longer leg is __________ and the hypotenuse is ___________.
2. In a 30-60-90 triangle, if the shorter leg is 12, then the longer leg is __________ and the hypotenuse is ___________.
3. In a 30-60-90 triangle, if the longer leg is 10, then the shorter leg is __________ and the hypotenuse is ___________.
4. In a 30-60-90 triangle, if the shorter leg is 16, then the longer leg is __________ and the hypotenuse is ___________.
5. In a 30-60-90 triangle, if the longer leg is 3, then the shorter leg is __________ and the hypotenuse is ___________.
6. In a 30-60-90 triangle, if the shorter leg is , then the longer leg is __________ and the hypotenuse is ___________.
7. In a 30-60-90 triangle, if the longer leg is , then the shorter leg is __________ and the hypotenuse is ___________.
8. A rectangle has sides of length 7 and . What is the length of the diagonal?
9. Two (opposite) sides of a rectangle are 15 and the diagonal is 30. What is the length of the other two sides?
10. What is the height of an equilateral triangle with sides of length 6 in?
11. What is the area of an equilateral triangle with sides of length 10 ft?
12. A regular hexagon has sides of length 3 in. What is the area of the hexagon?
13. The area of an equilateral triangle is . What is the length of a side?
14. If a road has a grade of , this means that its angle of elevation is . If you travel 3 miles on this road, how much elevation have you gained in feet (5280 ft = 1 mile)?
15. If a road has a grade of , this means that its angle of elevation is . If you travel x miles on this road, how much elevation have you gained in feet (5280 ft = 1 mile)?

To view the Explore More answers, open this PDF file and look for section 1.6.

Vocabulary Language: English

30-60-90 Triangle

30-60-90 Triangle

A 30-60-90 triangle is a special right triangle with angles of $30^\circ$, $60^\circ$, and $90^\circ$.
45-45-90 Triangle

45-45-90 Triangle

A 45-45-90 triangle is a special right triangle with angles of $45^\circ$, $45^\circ$, and $90^\circ$.