Intermediate Geometry : Acute / Obtuse Triangles

Study concepts, example questions & explanations for Intermediate Geometry

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Example Questions

Example Question #1 : Acute / Obtuse Triangles

For the triangle below, the perimeter is \displaystyle 40. Find the value of \displaystyle x.

1

Possible Answers:

\displaystyle 10

\displaystyle 15

\displaystyle 20

\displaystyle 5

Correct answer:

\displaystyle 10

Explanation:

The dashes on the two sides of the triangle indicate that those two sides are congruent. Thus, the length of the missing side is \displaystyle 2x-5 also.

Now, use the information given about the perimeter to set up an equation to solve for \displaystyle x. The sum of all the sides will give the perimeter.

\displaystyle 2x-5+2x-5+x=40

\displaystyle 5x-10=40

\displaystyle 5x=50

\displaystyle x=10

Example Question #1 : How To Find The Perimeter Of An Acute / Obtuse Triangle

Find the value of \displaystyle x if the perimeter is \displaystyle 24.

2

Possible Answers:

\displaystyle 12

\displaystyle 6

\displaystyle 9

\displaystyle 3

Correct answer:

\displaystyle 6

Explanation:

The dashes on the two sides of the triangle indicate that those two sides are congruent. Thus, the length of the missing side is \displaystyle x+3 also.

Now, use the information given about the perimeter to set up an equation to solve for \displaystyle x. The sum of all the sides will give the perimeter.

\displaystyle x+3+x+3+x=24

\displaystyle 3x+6=24

\displaystyle 3x=18

\displaystyle x=6

Example Question #2 : Acute / Obtuse Triangles

Find the value of \displaystyle x if the perimeter is \displaystyle 39.

3

Possible Answers:

\displaystyle 3

\displaystyle 7

\displaystyle 14

\displaystyle 21

Correct answer:

\displaystyle 7

Explanation:

The dashes on the two sides of the triangle indicate that those two sides are congruent. Thus, the length of the missing side is \displaystyle 2x-2 also.

Now, use the information given about the perimeter to set up an equation to solve for \displaystyle x. The sum of all the sides will give the perimeter.

\displaystyle 2x-2+2x-2+3x-6=39

\displaystyle 7x-10=39

\displaystyle 7x=49

\displaystyle x=7

Example Question #1 : How To Find The Perimeter Of An Acute / Obtuse Triangle

An acute isosceles triangle has an area of \displaystyle 24 square units and a base with length \displaystyle 6. Find the perimeter of this triangle. 

Possible Answers:

\displaystyle \sqrt{73}+8

\displaystyle 2\sqrt{73} +6

\displaystyle \sqrt{73}+6

\displaystyle 2\sqrt{73}+8

Correct answer:

\displaystyle 2\sqrt{73} +6

Explanation:

To solve this problem, first work backwards using the formula: 

\displaystyle Area=\frac{b\cdot h}{2}


Plugging in the given information and solving for height.

\displaystyle 24=\frac{6h}{2}

\displaystyle 24\times2=6h

\displaystyle 48=6h

\displaystyle h=\frac{48}{6}=8

Now that you've found the height of the triangle, use the Pythagorean Theorem to find the length of one of the equivalent sides of the triangle. 

(Note, when applying the formula divide the measurement of the base in half--so the triangle will have sides of \displaystyle 3, 8 and \displaystyle c^2

Thus, the solution is:
\displaystyle 3^2+8^2=c^2
\displaystyle 9+64=c^2
\displaystyle 73=c^2

\displaystyle c=\sqrt{73}

In this problem, the isosceles triangle will have two sides with the length of \displaystyle \sqrt{73} and one side length of \displaystyle 6. Therefore, the perimeter is: \displaystyle 2\sqrt{73}+6

Example Question #1 : Acute / Obtuse Triangles

A scalene triangle has sides with lengths of \displaystyle \frac{3}{4} foot, \displaystyle \frac{1}{4} foot, and \displaystyle \frac{2}{6} foot. Find the perimeter of the the triangle (in inches). 

Possible Answers:

\displaystyle 12in

\displaystyle 18in

\displaystyle 16in

\displaystyle 24in

Correct answer:

\displaystyle 16in

Explanation:

To solve this problem, first convert each of the fractions from feet to inches. 

\displaystyle \frac{3}{4}=\frac{9}{12}=9 inches


\displaystyle \frac{1}{4}=\frac{3}{12}=3 inches


\displaystyle \frac{2}{6}=\frac{4}{12}=4 inches

The perimeter of the triangle must equal \displaystyle 9+3+4=16 

Example Question #3 : How To Find The Perimeter Of An Acute / Obtuse Triangle

An acute triangle has side lengths of \displaystyle 12 and \displaystyle 15. The hypotenuse of the triangle is not \displaystyle 15. Find the perimeter of this triangle.  

Possible Answers:

\displaystyle 3\sqrt{41}

\displaystyle \sqrt{41}+27

\displaystyle 41

\displaystyle 3\sqrt{41}+27

Correct answer:

\displaystyle 3\sqrt{41}+27

Explanation:

Since the two side lengths provided in the question are not the hypotenuse, first use the Pythagorean Theorem to find the length of the third side. 

\displaystyle a^2+b^2=c^2

\displaystyle 12^2+15^2=c^2

\displaystyle 144+225=c^2

\displaystyle c^2=369

\displaystyle c=\sqrt{369}=\sqrt{9}\sqrt{41}=3\sqrt{41}

The perimeter is equal to \displaystyle a+b+c.

\displaystyle 12+15+3\sqrt{41}

\displaystyle 3\sqrt{41}+27

Example Question #4 : How To Find The Perimeter Of An Acute / Obtuse Triangle

Inter_geo_tri_series_

Find the perimeter of the triangle shown above.

Possible Answers:

\displaystyle 4.5+\sqrt{26.5}in

\displaystyle \sqrt{26.5}in

\displaystyle \sqrt{26.5+7}in

\displaystyle 7+\sqrt{26.5}in

Correct answer:

\displaystyle 7+\sqrt{26.5}in

Explanation:

Since the two side lengths provided in the question are not the hypotenuse, first use the Pythagorean Theorem to find the length of the third side. 

\displaystyle a^2+b^2=c^2


\displaystyle 2.5^2+4.5^2=c^2


\displaystyle 6.25+20.25=c^2

\displaystyle c^2=26.5

\displaystyle c=\sqrt{26.5}


Perimeter is equal to \displaystyle a+b+c.

\displaystyle 2.5+4.5+\sqrt{26.5}=7+ \sqrt{26.5}

Example Question #1 : How To Find The Perimeter Of An Acute / Obtuse Triangle

An obtuse Isosceles triangle has an area of \displaystyle 32 square units and a base with length \displaystyle 8. Find the perimeter of this triangle.

Possible Answers:

\displaystyle 8+2\sqrt{5}

\displaystyle 8+8\sqrt{5}

\displaystyle 4\sqrt{5}

\displaystyle 4+\sqrt{5}

Correct answer:

\displaystyle 8+8\sqrt{5}

Explanation:

To solve this solution, first work backwards using the formula: \displaystyle Area=\frac{b\cdot h}{2}

\displaystyle 32=\frac{8h}{2}

\displaystyle 8h=64

\displaystyle h=8

Now that you've found the height of the triangle, use the Pythagorean Theorem to find the length of one of the equivalent sides of the triangle. 

(Note, when applying the formula divide the measurement of the base in half--so the triangle will have sides of \displaystyle 4,8 and \displaystyle c^2

Thus, the solution is:
\displaystyle 4^2+8^2=c^2

\displaystyle 16+64=c^2

\displaystyle c^2=80

\displaystyle c=\sqrt{80}=4\sqrt{5}

In this problem, the isosceles triangle will have two sides with the length of \displaystyle 4\sqrt{5} and one side length of \displaystyle 8. Therefore, the perimeter is: \displaystyle 8+8\sqrt{5}.

Example Question #6 : How To Find The Perimeter Of An Acute / Obtuse Triangle

Inter_geo_tri_series_

Find the perimeter of the acute isosceles triangle shown above. Note, the triangle has a base of \displaystyle 14 and height of \displaystyle 20.

Possible Answers:

\displaystyle 14\sqrt{449}

\displaystyle 14+2\sqrt{449}

\displaystyle 14+\sqrt{449}

\displaystyle \sqrt{449}

Correct answer:

\displaystyle 14+2\sqrt{449}

Explanation:

Use the Pythagorean Theorem to find the length of one of the equivalent sides of the triangle. 

(Note, when applying the formula divide the measurement of the base in half--so the triangle will have sides of \displaystyle 7,20 and \displaystyle c^2

Thus, the solution is:
\displaystyle 7^2+20^2=c^2

\displaystyle 49+400=c^2

\displaystyle c^2=449

\displaystyle c=\sqrt{449}

In this problem, the isosceles triangle will have two sides with the length of \displaystyle \sqrt{449} and one side length of \displaystyle 14. Therefore, the perimeter is: \displaystyle 14+2\sqrt{449}.

Example Question #3 : Acute / Obtuse Triangles

Find the perimeter of the triangle below. Round to the nearest tenths place.

4

Possible Answers:

\displaystyle 22.4

\displaystyle 24.5

\displaystyle 30.7

\displaystyle 25.3

Correct answer:

\displaystyle 22.4

Explanation:

Draw in the height to create a right triangle.

4a

Now, using the relationship between the lengths of sides in a \displaystyle 30-60-90 triangle, where the long leg is the length of the short leg times \displaystyle \sqrt3 and the hypotenuse is two times the length of the short side. We can find out that the height of the triangle is \displaystyle 3 since it is the short leg and the hypotenuse is \displaystyle 6.

The dashes on two sides of the triangle indicate that these two sides are congruent. The three side lengths of the triangle are \displaystyle 6, 6, \text{ and }6\sqrt3.

Now, add up these side lengths to find the perimeter.

\displaystyle \text{Perimeter}=6+6+6\sqrt3=22.4

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