Intermediate Geometry : Acute / Obtuse Isosceles Triangles

Study concepts, example questions & explanations for Intermediate Geometry

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Example Questions

Example Question #1 : Triangles

An isosceles triangle has a base of 12\ cm\displaystyle 12\ cm and an area of 42\ cm^{2}\displaystyle 42\ cm^{2}. What must be the height of this triangle?

Possible Answers:

8\ cm\displaystyle 8\ cm

7\ cm\displaystyle 7\ cm

6\ cm\displaystyle 6\ cm

10\ cm\displaystyle 10\ cm

9\ cm\displaystyle 9\ cm

Correct answer:

7\ cm\displaystyle 7\ cm

Explanation:

A=\frac{1}{2}bh\displaystyle A=\frac{1}{2}bh

6x=42\displaystyle 6x=42

x=7\displaystyle x=7

Example Question #1 : Triangles

An isosceles triangle has a perimeter of \displaystyle 22. If the base of the triangle is two less than two times the length of each leg, what is the height of the triangle?

Possible Answers:

\displaystyle 4.82

\displaystyle 3.32

\displaystyle 3.11

The height of the triangle cannot be determined with the given information.

Correct answer:

\displaystyle 3.32

Explanation:

First, find the lengths of the triangle.

Let \displaystyle x be the length of each leg. Then, the length of the base must be \displaystyle 2x-2.

Use the information given about the perimeter to solve for \displaystyle x.

\displaystyle x+x+2x-2=22

\displaystyle 4x-2=22

\displaystyle 4x=24

\displaystyle x=6

Plug this value in to find the length of the base.

\displaystyle \text{Length of Base}=2(6)-2=10

Now, recall that the height of an isosceles triangle can split the entire triangle into two congruent right triangle as shown by the figure below.

13

Thus, we can use the Pythagorean Theorem to find the length of the height.

\displaystyle \text{Height}=\sqrt{\text{leg}^2-\text{half of base}^2}

Plug in the given values to find the height of the triangle.

\displaystyle \text{Height}=\sqrt{6^2-5^2}=\sqrt{11}=3.32

Make sure to round to \displaystyle 2 places after the decimal.

 

Example Question #1 : Acute / Obtuse Isosceles Triangles

An isosceles triangle has a perimeter of \displaystyle 32. If the base of the triangle is \displaystyle 18 less than three times the length of a leg, what is the height of the triangle?

Possible Answers:

\displaystyle 4

\displaystyle 8

\displaystyle 9

\displaystyle 6

Correct answer:

\displaystyle 8

Explanation:

First, find the lengths of the triangle.

Let \displaystyle x be the length of each leg. Then, the length of the base must be \displaystyle 3x-18.

Use the information given about the perimeter to solve for \displaystyle x.

\displaystyle x+x+3x-18=32

\displaystyle 5x-18=32

\displaystyle 5x=50

\displaystyle x=10

Plug this value in to find the length of the base.

\displaystyle \text{Length of Base}=3(10)-18=12

Now, recall that the height of an isosceles triangle can split the entire triangle into two congruent right triangle as shown by the figure below.

13

Thus, we can use the Pythagorean Theorem to find the length of the height.

\displaystyle \text{Height}=\sqrt{\text{leg}^2-\text{half of base}^2}

Plug in the given values to find the height of the triangle.

\displaystyle \text{Height}=\sqrt{10^2-6^2}=\sqrt{64}=8

Make sure to round to \displaystyle 2 places after the decimal.

 

Example Question #3 : Triangles

An isosceles triangle has a perimeter of \displaystyle 19. If the length of the base is one less than twice the length of a leg, what is the height of the triangle?

Possible Answers:

\displaystyle 2.90

\displaystyle 1.29

\displaystyle 2.44

\displaystyle 2.18

Correct answer:

\displaystyle 2.18

Explanation:

First, find the lengths of the triangle.

Let \displaystyle x be the length of each leg. Then, the length of the base must be \displaystyle 2x-1.

Use the information given about the perimeter to solve for \displaystyle x.

\displaystyle x+x+2x-1=19

\displaystyle 4x-1=19

\displaystyle 4x=20

\displaystyle x=5

Plug this value in to find the length of the base.

\displaystyle \text{Length of Base}=2(5)-1=9

Now, recall that the height of an isosceles triangle can split the entire triangle into two congruent right triangle as shown by the figure below.

13

Thus, we can use the Pythagorean Theorem to find the length of the height.

\displaystyle \text{Height}=\sqrt{\text{leg}^2-\text{half of base}^2}

Plug in the given values to find the height of the triangle.

\displaystyle \text{Height}=\sqrt{5^2-4.5^2}=\sqrt{4.75}=2.18

Make sure to round to \displaystyle 2 places after the decimal.

 

Example Question #1 : Triangles

An isosceles triangle has a perimeter of \displaystyle 16. If the length of the base is fourteen less than three times the length of a leg, what is the height of the triangle?

Possible Answers:

\displaystyle 4.93

\displaystyle 5.66

\displaystyle 5.89

\displaystyle 5.09

Correct answer:

\displaystyle 5.66

Explanation:

First, find the lengths of the triangle.

Let \displaystyle x be the length of each leg. Then, the length of the base must be \displaystyle 3x-14.

Use the information given about the perimeter to solve for \displaystyle x.

\displaystyle x+x+3x-14=16

\displaystyle 5x-14=16

\displaystyle 5x=30

\displaystyle x=6

Plug this value in to find the length of the base.

\displaystyle \text{Length of Base}=3(6)-14=4

Now, recall that the height of an isosceles triangle can split the entire triangle into two congruent right triangle as shown by the figure below.

13

Thus, we can use the Pythagorean Theorem to find the length of the height.

\displaystyle \text{Height}=\sqrt{\text{leg}^2-\text{half of base}^2}

Plug in the given values to find the height of the triangle.

\displaystyle \text{Height}=\sqrt{6^2-2^2}=\sqrt{32}=5.66

Make sure to round to \displaystyle 2 places after the decimal.

 

Example Question #1 : Acute / Obtuse Isosceles Triangles

An isosceles triangle has a perimeter of \displaystyle 19. If the length of the base is two less than one-third of the length of a leg, what is the height of the triangle?

Possible Answers:

\displaystyle 8.19

\displaystyle 8.99

\displaystyle 8.82

\displaystyle 9.02

Correct answer:

\displaystyle 8.99

Explanation:

First, find the lengths of the triangle.

Let \displaystyle x be the length of each leg. Then, the length of the base must be \displaystyle \frac{1}{3}x-2.

Use the information given about the perimeter to solve for \displaystyle x.

\displaystyle x+x+\frac{1}{3}x-2=19

\displaystyle \frac{7}{3}x-2=19

\displaystyle \frac{7}{3}x=21

\displaystyle x=9

Plug this value in to find the length of the base.

\displaystyle \text{Length of Base}=\frac{1}{3}(9)-2=1

Now, recall that the height of an isosceles triangle can split the entire triangle into two congruent right triangle as shown by the figure below.

13

Thus, we can use the Pythagorean Theorem to find the length of the height.

\displaystyle \text{Height}=\sqrt{\text{leg}^2-\text{half of base}^2}

Plug in the given values to find the height of the triangle.

\displaystyle \text{Height}=\sqrt{9^2-(\frac{1}{2})^2}=\sqrt{80.75}=8.99

Make sure to round to \displaystyle 2 places after the decimal.

 

Example Question #6 : Triangles

An isosceles triangle has a perimeter of \displaystyle 38. If the length of the base is ten less than twice the length of a leg, what is the height of the triangle?

Possible Answers:

\displaystyle 9.87

\displaystyle 10.12

\displaystyle 9.75

\displaystyle 8.90

Correct answer:

\displaystyle 9.75

Explanation:

First, find the lengths of the triangle.

Let \displaystyle x be the length of each leg. Then, the length of the base must be \displaystyle 2x-10.

Use the information given about the perimeter to solve for \displaystyle x.

\displaystyle x+x+2x-10=38

\displaystyle 4x-10=38

\displaystyle 4x=48

\displaystyle x=12

Plug this value in to find the length of the base.

\displaystyle \text{Length of Base}=2(12)-10=14

Now, recall that the height of an isosceles triangle can split the entire triangle into two congruent right triangle as shown by the figure below.

13

Thus, we can use the Pythagorean Theorem to find the length of the height.

\displaystyle \text{Height}=\sqrt{\text{leg}^2-\text{half of base}^2}

Plug in the given values to find the height of the triangle.

\displaystyle \text{Height}=\sqrt{12^2-7^2}=\sqrt{95}=9.75

Make sure to round to \displaystyle 2 places after the decimal.

 

Example Question #7 : Triangles

An isosceles triangle has a perimeter of \displaystyle 11. If the length of the base is four more than one-third of the length of a leg, what is the height of the triangle?

Possible Answers:

\displaystyle 1.97

\displaystyle 1.20

\displaystyle 1.59

\displaystyle 1.66

Correct answer:

\displaystyle 1.66

Explanation:

First, find the lengths of the triangle.

Let \displaystyle x be the length of each leg. Then, the length of the base must be \displaystyle \frac{1}{3}x+4.

Use the information given about the perimeter to solve for \displaystyle x.

\displaystyle x+x+\frac{1}{3}x+4=11

\displaystyle \frac{7}{3}x+4=11

\displaystyle \frac{7}{3}x=7

\displaystyle x=3

Plug this value in to find the length of the base.

\displaystyle \text{Length of Base}=\frac{1}{3}(3)+4=5

Now, recall that the height of an isosceles triangle can split the entire triangle into two congruent right triangle as shown by the figure below.

13

Thus, we can use the Pythagorean Theorem to find the length of the height.

\displaystyle \text{Height}=\sqrt{\text{leg}^2-\text{half of base}^2}

Plug in the given values to find the height of the triangle.

\displaystyle \text{Height}=\sqrt{3^2-(\frac{5}{2})^2}=\sqrt{2.75}=1.66

Make sure to round to \displaystyle 2 places after the decimal.

 

Example Question #8 : Triangles

The perimeter of an isosceles triangle is \displaystyle 50. If the length of the base is five more than one-fourth the length of a leg, what is the height of the triangle?

Possible Answers:

\displaystyle 18.45

\displaystyle 15.20

\displaystyle 19.36

\displaystyle 19.01

Correct answer:

\displaystyle 19.36

Explanation:

First, find the lengths of the triangle.

Let \displaystyle x be the length of each leg. Then, the length of the base must be \displaystyle \frac{1}{4}x+5.

Use the information given about the perimeter to solve for \displaystyle x.

\displaystyle x+x+\frac{1}{4}x+5=50

\displaystyle \frac{9}{4}x+5=50

\displaystyle \frac{9}{4}x=45

\displaystyle x=20

Plug this value in to find the length of the base.

\displaystyle \text{Length of Base}=\frac{1}{4}(20)+5=10

Now, recall that the height of an isosceles triangle can split the entire triangle into two congruent right triangle as shown by the figure below.

13

Thus, we can use the Pythagorean Theorem to find the length of the height.

\displaystyle \text{Height}=\sqrt{\text{leg}^2-\text{half of base}^2}

Plug in the given values to find the height of the triangle.

\displaystyle \text{Height}=\sqrt{20^2-5^2}=\sqrt{375}=19.36

Make sure to round to \displaystyle 2 places after the decimal.

 

Example Question #1 : Triangles

The perimeter of an isosceles triangle is \displaystyle 44. If the length of the base is ten more than one-eighth the length of a leg, what is the height of the triangle?

Possible Answers:

\displaystyle 15.28

\displaystyle 14.83

\displaystyle 13.41

\displaystyle 10.21

Correct answer:

\displaystyle 14.83

Explanation:

First, find the lengths of the triangle.

Let \displaystyle x be the length of each leg. Then, the length of the base must be \displaystyle \frac{1}{8}x+10.

Use the information given about the perimeter to solve for \displaystyle x.

\displaystyle x+x+\frac{1}{8}x+10=44

\displaystyle \frac{17}{8}x+10=44

\displaystyle \frac{17}{8}x=34

\displaystyle x=16

Plug this value in to find the length of the base.

\displaystyle \text{Length of Base}=\frac{1}{8}(16)+10=12

Now, recall that the height of an isosceles triangle can split the entire triangle into two congruent right triangle as shown by the figure below.

13

Thus, we can use the Pythagorean Theorem to find the length of the height.

\displaystyle \text{Height}=\sqrt{\text{leg}^2-\text{half of base}^2}

Plug in the given values to find the height of the triangle.

\displaystyle \text{Height}=\sqrt{16^2-6^2}=\sqrt{220}=14.83

Make sure to round to \displaystyle 2 places after the decimal.

 

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