Intermediate Geometry : Triangles

Study concepts, example questions & explanations for Intermediate Geometry

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Example Questions

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

 is an equilateral triangle;  is an equiangular triangle.

True or false: From the given information, it follows that .

Possible Answers:

False

True

Correct answer:

True

Explanation:

As we are establishing whether or not , then , and  correspond respectively to , and .

A triangle is equilateral (having three sides of the same length) if and only if it is also equiangular (having three angles of the same measure, each of which is  ). It follows that all angles of both triangles measure .

Specifically,  and , making two pairs of corresponding angles congruent. By the Angle-Angle Similarity Postulate, it follows that , making the correct answer "true".

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

Given:  and  such that 

Which statement(s) must be true?

(a) 

(b) 

Possible Answers:

(a) but not (b)

(b) but not (a)

(a) and (b)

Neither (a) nor (b)

Correct answer:

(a) but not (b)

Explanation:

The sum of the measures of the interior angles of a triangle is , so

Also,

,

so

By similar reasoning, it holds that

Since , by substitution,

Therefore, 

,

or 

This, along with the statement that , sets up the conditions of the Angle-Angle Similarity Postulate - if two angles of one triangle are congruent to the two corresponding angles of another triangle, the two triangles are similar. It follows that 

.

However, congruence cannot be proved, since at least one side congruence is needed to prove this. This is not given in the problem. 

Therefore, statement (a) must hold, but not necessarily statement (b).

Example Question #1 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

An isoceles triangle has a vertex angle that is twenty more than twice the base angle.  What is the difference between the vertex and base angles?

Possible Answers:

Correct answer:

Explanation:

A triangle has degrees.  An isoceles triangle has one vertex angle and two congruent base angles.

Let = the base angle and  = vertex angle

So the equation to solve becomes 

or

 

so the base angle is and the vertex angle is and the difference is .

Example Question #1 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

An ssosceles triangle has interior angles of  degrees and  degrees. Find the missing angle. 

Possible Answers:

 

 

 

 

Correct answer:

 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of  degrees. 

Thus, the solution is:





Example Question #3 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

The largest angle in an obtuse isosceles triangle is  degrees. Find the measurement of one of the two equivalent interior angles. 

Possible Answers:

 

 

 

 

Correct answer:

 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of  degrees. Since this is an obtuse isosceles triangle, the two missing angles must be acute angles.  

Thus, the solution is:



Example Question #1 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

The two equivalent interior angles of an obtuse isosceles triangle each have a measurement of  degrees. Find the measurement of the obtuse angle. 

Possible Answers:

 

 

 

 

Correct answer:

 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of  degrees.

Thus, the solution is:



Example Question #1 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

In an obtuse isosceles triangle the angle measurements are, , and . Find the measurement of one of the acute angles. 

Possible Answers:

 

 

 

 

 

Correct answer:

 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of  degrees. Since this is an obtuse isosceles triangle, the two missing angles must be acute angles. 

The solution is:



However,  degrees is the measurement of both of the acute angles combined.

Each individual angle is .

Example Question #6 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

In an acute isosceles triangle the two equivalent interior angles each have a measurement of  degrees. Find the missing angle. 

Possible Answers:

 

 

 

 

Correct answer:

 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of  degrees. Since this is an acute isosceles triangle, all of the interior angles must be acute angles.

The solution is:



 

Example Question #7 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

In an acute isosceles triangle the two equivalent interior angles are each  degrees. Find the missing angle. 

Possible Answers:

 

 

 

 

Correct answer:

 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of  degrees. Since this is an acute isosceles triangle, all of the interior angles must be acute angles.

The solution is:



Example Question #8 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

The largest angle in an obtuse isosceles triangle is  degrees. Find the measurement of one of the equivalent interior angles. 

Possible Answers:

 

 

 

 

 

Correct answer:

 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of  degrees. Since this is an obtuse Isosceles triangle, the two missing angles must be acute angles.  

Thus, the solution is:



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