Intermediate Geometry : Triangles

Study concepts, example questions & explanations for Intermediate Geometry

varsity tutors app store varsity tutors android store

Example Questions

Example Question #8 : How To Find The Area Of An Acute / Obtuse Isosceles Triangle

An isosceles triangle is placed in a circle as shown by the figure below.

1

If the diameter of the circle is \(\displaystyle 18\), find the area of the shaded region.

Possible Answers:

\(\displaystyle 173.47\)

\(\displaystyle 198.52\)

\(\displaystyle 174.55\)

\(\displaystyle 189.62\)

Correct answer:

\(\displaystyle 173.47\)

Explanation:

2

From the given image, you should notice that the base of the triangle is also the diameter of the circle. In addition, the height of the triangle is also the radius of the circle.

Thus, we can find the area of the triangle.

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(\text{base}\times\text{height})\)

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(18 \times 9)=81\)

Next, recall how to find the area of a circle.

\(\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2\)

\(\displaystyle \text{Area of Circle}=\pi\times 9^2= 81\pi\)

To find the area of the shaded region, subtract the two areas.

\(\displaystyle \text{Area of Shaded Region}=\text{Area of Circle}-\text{Area of Triangle}\)

\(\displaystyle \text{Area of Shaded Region}=81\pi-81=173.47\)

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

Example Question #9 : How To Find The Area Of An Acute / Obtuse Isosceles Triangle

An isosceles triangle is placed in a circle as shown by the figure below.

1

If the diameter of the circle is \(\displaystyle 26\), find the area of the shaded region.

Possible Answers:

\(\displaystyle 401.25\)

\(\displaystyle 355.21\)

\(\displaystyle 372.90\)

\(\displaystyle 361.93\)

Correct answer:

\(\displaystyle 361.93\)

Explanation:

2

From the given image, you should notice that the base of the triangle is also the diameter of the circle. In addition, the height of the triangle is also the radius of the circle.

Thus, we can find the area of the triangle.

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(\text{base}\times\text{height})\)

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(26 \times 13)=169\)

Next, recall how to find the area of a circle.

\(\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2\)

\(\displaystyle \text{Area of Circle}=\pi\times 13^2= 169\pi\)

To find the area of the shaded region, subtract the two areas.

\(\displaystyle \text{Area of Shaded Region}=\text{Area of Circle}-\text{Area of Triangle}\)

\(\displaystyle \text{Area of Shaded Region}=169\pi-169=361.93\)

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

Example Question #10 : How To Find The Area Of An Acute / Obtuse Isosceles Triangle

An isosceles triangle is placed in a circle as shown by the figure below.

1

If the diameter of the circle is \(\displaystyle 44\), find the area of the shaded region.

Possible Answers:

\(\displaystyle 1121.77\)

\(\displaystyle 980.27\)

\(\displaystyle 1092.14\)

\(\displaystyle 1036.53\)

Correct answer:

\(\displaystyle 1036.53\)

Explanation:

2

From the given image, you should notice that the base of the triangle is also the diameter of the circle. In addition, the height of the triangle is also the radius of the circle.

Thus, we can find the area of the triangle.

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(\text{base}\times\text{height})\)

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(44 \times 22)=484\)

Next, recall how to find the area of a circle.

\(\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2\)

\(\displaystyle \text{Area of Circle}=\pi\times 22^2= 484\pi\)

To find the area of the shaded region, subtract the two areas.

\(\displaystyle \text{Area of Shaded Region}=\text{Area of Circle}-\text{Area of Triangle}\)

\(\displaystyle \text{Area of Shaded Region}=484\pi-484=1036.53\)

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

Example Question #11 : How To Find The Area Of An Acute / Obtuse Isosceles Triangle

An isosceles triangle is placed in a circle as shown by the figure below.

1

If the diameter of the circle is \(\displaystyle 46\), find the area of the shaded region.

Possible Answers:

\(\displaystyle 1132.90\)

\(\displaystyle 1190.02\)

\(\displaystyle 1098.98\)

\(\displaystyle 1145.52\)

Correct answer:

\(\displaystyle 1132.90\)

Explanation:

2

From the given image, you should notice that the base of the triangle is also the diameter of the circle. In addition, the height of the triangle is also the radius of the circle.

Thus, we can find the area of the triangle.

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(\text{base}\times\text{height})\)

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(46 \times 23)=529\)

Next, recall how to find the area of a circle.

\(\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2\)

\(\displaystyle \text{Area of Circle}=\pi\times 23^2= 529\pi\)

To find the area of the shaded region, subtract the two areas.

\(\displaystyle \text{Area of Shaded Region}=\text{Area of Circle}-\text{Area of Triangle}\)

\(\displaystyle \text{Area of Shaded Region}=529\pi-529=1132.90\)

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

Example Question #12 : How To Find The Area Of An Acute / Obtuse Isosceles Triangle

An isosceles triangle is placed in a circle as shown by the figure below.

1

If the diameter of the circle is \(\displaystyle 30\), find the area of the shaded region.

Possible Answers:

\(\displaystyle 402.08\)

\(\displaystyle 498.20\)

\(\displaystyle 481.86\)

\(\displaystyle 441.12\)

Correct answer:

\(\displaystyle 481.86\)

Explanation:

2

From the given image, you should notice that the base of the triangle is also the diameter of the circle. In addition, the height of the triangle is also the radius of the circle.

Thus, we can find the area of the triangle.

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(\text{base}\times\text{height})\)

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(30 \times 15)=225\)

Next, recall how to find the area of a circle.

\(\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2\)

\(\displaystyle \text{Area of Circle}=\pi\times 15^2= 225\pi\)

To find the area of the shaded region, subtract the two areas.

\(\displaystyle \text{Area of Shaded Region}=\text{Area of Circle}-\text{Area of Triangle}\)

\(\displaystyle \text{Area of Shaded Region}=225\pi-225=481.86\)

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

Example Question #13 : How To Find The Area Of An Acute / Obtuse Isosceles Triangle

An isosceles triangle is placed in a circle as shown by the figure below.

1

If the diameter of the circle is \(\displaystyle 52\), find the area of the shaded region.

Possible Answers:

\(\displaystyle 1181.21\)

\(\displaystyle 1190.21\)

\(\displaystyle 1098.21\)

\(\displaystyle 1447.72\)

Correct answer:

\(\displaystyle 1447.72\)

Explanation:

2

From the given image, you should notice that the base of the triangle is also the diameter of the circle. In addition, the height of the triangle is also the radius of the circle.

Thus, we can find the area of the triangle.

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(\text{base}\times\text{height})\)

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(52 \times 26)=676\)

Next, recall how to find the area of a circle.

\(\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2\)

\(\displaystyle \text{Area of Circle}=\pi\times 26^2= 676\pi\)

To find the area of the shaded region, subtract the two areas.

\(\displaystyle \text{Area of Shaded Region}=\text{Area of Circle}-\text{Area of Triangle}\)

\(\displaystyle \text{Area of Shaded Region}=676\pi-676=1447.72\)

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

Example Question #14 : How To Find The Area Of An Acute / Obtuse Isosceles Triangle

A triangle is placed in a parallelogram so that they share a base.

9

If the height of the triangle is half the height of the parallelogram, find the area of the shaded region.

Possible Answers:

\(\displaystyle 0.75\)

\(\displaystyle 1\)

\(\displaystyle 1.25\)

\(\displaystyle 1.5\)

Correct answer:

\(\displaystyle 1.5\)

Explanation:

13

In order to find the area of the shaded region, we will need to find the areas of the triangle and of the parallelogram.

First, recall how to find the area of a parallelogram.

\(\displaystyle \text{Area of Parallelogram}=\text{base}\times\text{height}\)

\(\displaystyle \text{Area of Parallelogram}=2\times 1 = 2\)

Next, recall how to find the area of a triangle.

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(\text{base}\times\text{height})\)

Now, find the height of the triangle.

\(\displaystyle \text{Height of Triangle}=\frac{\text{Height of Parallelogram}}{2}\)

\(\displaystyle \text{Height of Triangle}=\frac{1}{2}\)

Plug this value in to find the area of the triangle.

\(\displaystyle \text{Area of Triangle}=\frac{1}{2}(2\times \frac{1}{2})=\frac{1}{2}=0.5\)

Subtract the two areas to find the area of the shaded region.

\(\displaystyle \text{Area of Shaded Region}=\text{Area of Parallelogram}-\text{Area of Triangle}\)

\(\displaystyle \text{Area of Shaded Region}=2-0.5=1.5\)

Example Question #1 : How To Find If Of Acute / Obtuse Isosceles Triangle Are Similar

Parallel 2

Refer to the above diagram. \(\displaystyle m ||n\).

True or false: From the information given, it follows that \(\displaystyle \bigtriangleup AVB \sim \bigtriangleup CVD\).

Possible Answers:

False

True

Correct answer:

True

Explanation:

By the Angle-Angle Similarity Postulate, if two pairs of corresponding angles of a triangle are congruent, the triangles themselves are similar. 

\(\displaystyle \angle AVB\) and \(\displaystyle \angle CVD\) are a pair of vertical angles, having the same vertex and having sides opposite each other. As such, \(\displaystyle \angle AVB \cong \angle CVD\).

\(\displaystyle \angle ABV\) and \(\displaystyle \angle CDV\) are alternating interior angles formed by two parallel lines \(\displaystyle m\) and \(\displaystyle n\) cut by a transversal \(\displaystyle t\). As a consequence, \(\displaystyle \angle ABV \cong \angle CDV\).

The conditions of the Angle-Angle Similarity Postulate are satisfied, and it holds that \(\displaystyle \bigtriangleup AVB \sim \bigtriangleup CVD\).

Example Question #2 : How To Find If Of Acute / Obtuse Isosceles Triangle Are Similar

Parallel 2

Refer to the above diagram. \(\displaystyle \angle ABV \cong \angle DCV\).

True or false: From the information given, it follows that \(\displaystyle \bigtriangleup AVB \sim \bigtriangleup CVD\).

Possible Answers:

False

True

Correct answer:

False

Explanation:

The given information is actually inconclusive.

By the Angle-Angle Similarity Postulate, if two pairs of corresponding angles of a triangle are congruent, the triangles themselves are similar. Therefore, we seek to prove two of the following three angle congruence statements:

\(\displaystyle \angle AVB \cong \angle CVD\)

\(\displaystyle \angle ABV \cong \angle CDV\)

\(\displaystyle \angle BAV \cong \angle DCV\)

\(\displaystyle \angle AVB\) and \(\displaystyle \angle CVD\) are a pair of vertical angles, having the same vertex and having sides opposite each other. As such, \(\displaystyle \angle AVB \cong \angle CVD\)

\(\displaystyle \angle ABV \cong \angle DCV\), but this is not one of the statements we need to prove. Also, without further information - for example, whether \(\displaystyle m\) and \(\displaystyle n\) are parallel, which is not given to us - we have no way to prove either of the other two necessary statements. 

The correct response is "false".

Example Question #3 : How To Find If Of Acute / Obtuse Isosceles Triangle Are Similar

\(\displaystyle \bigtriangleup ABC\) and \(\displaystyle \bigtriangleup DEF\) are both isosceles triangles;

\(\displaystyle m \angle B = 120 ^{\circ }\)

\(\displaystyle m \angle F = 30 ^{\circ }\)

True or false: from the given information, it follows that \(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\).

Possible Answers:

False

True

Correct answer:

False

Explanation:

As we are establishing whether or not \(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\), then \(\displaystyle A\)\(\displaystyle B\), and \(\displaystyle C\) correspond respectively to \(\displaystyle D\)\(\displaystyle E\), and \(\displaystyle F\).

\(\displaystyle \bigtriangleup DEF\) is an isosceles triangle, so it must have two congruent angles. \(\displaystyle \angle F\) has measure \(\displaystyle 30^{\circ }\), so either \(\displaystyle \angle D\) has this measure, \(\displaystyle \angle E\) has this measure, or \(\displaystyle \angle D \cong \angle E\). If we examine the second case, it immediately follows that \(\displaystyle \angle B \ncong \angle E\). One condition of the similarity of triangles is that all pairs of corresponding angles be congruent; since there is at least one case that violates this condition, it does not necessarily follow that  \(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\). This makes the correct response "false".

Learning Tools by Varsity Tutors