ISEE Upper Level Math : How to find an angle in an acute / obtuse triangle

Study concepts, example questions & explanations for ISEE Upper Level Math

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Example Questions

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Example Question #1 : Acute / Obtuse Triangles

Which of the following is true about a triangle with two angles that measure  and ?

Possible Answers:

This triangle is scalene and right.

This triangle is isosceles and right.

This triangle is isosceles and obtuse.

This triangle cannot exist.

This triangle is scalene and obtuse.

Correct answer:

This triangle cannot exist.

Explanation:

A triangle must have at least two acute angles; however, a triangle with angles that measure  and  could have at most one acute angle, an impossible situation. Therefore, this triangle is nonexistent.

Example Question #5 : Acute / Obtuse Triangles

Which of the following is true about a triangle with two angles that measure  each?

Possible Answers:

The triangle cannot exist.

The triangle is obtuse and scalene.

The triangle is obtuse and isosceles.

The triangle is acute and isosceles.

The triangle is acute and scalene.

Correct answer:

The triangle cannot exist.

Explanation:

A triangle must have at least two acute angles; however, a triangle with angles that measure  would have two obtuse angles and at most one acute angle. This is not possible, so this triangle cannot exist.

Example Question #84 : Plane Geometry

One angle of an isosceles triangle has measure . What are the measures of the other two angles?

Possible Answers:

Not enough information is given to answer this question.

Correct answer:

Explanation:

An isosceles triangle not only has two sides of equal measure, it has two angles of equal measure. This means one of two things, which we examine separately:

Case 1: It has another  angle. This is impossible, since a triangle cannot have two obtuse angles.

Case 2: Its other two angles are the ones that are of equal measure. If we let  be their common measure, then, since the sum of the measures of a triangle is 

Both angles measure 

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

The angles of a triangle measure . Evaluate 

Possible Answers:

Correct answer:

Explanation:

The sum of the degree measures of the angles of a triangle is 180, so we solve for  in the following equation:

Example Question #3 : Acute / Obtuse Triangles

The acute angles of a right triangle measure  and 

Evaluate .

Possible Answers:

Correct answer:

Explanation:

The degree measures of the acute angles of a right triangle total 90, so we solve for  in the following equation:

Example Question #4 : Acute / Obtuse Triangles

Chords

Note: Figure NOT drawn to scale

Refer to the above figure. .

What is the measure of  ?

Possible Answers:

Correct answer:

Explanation:

Congruent chords of a circle have congruent minor arcs, so since , and their common measure is .

Since there are  in a circle, 

The inscribed angle  intercepts this arc and therefore has one-half its degree measure, which is 

Example Question #2 : How To Find An Angle In An Acute / Obtuse Triangle

Solve for :
Question11

Possible Answers:

Correct answer:

Explanation:

The sum of the internal angles of a triangle is equal to . Therefore:

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

Triangle 2

Refer to the above figure. Express  in terms of .

Possible Answers:

Correct answer:

Explanation:

The measure of an interior angle of a triangle is equal to 180 degrees minus that of its adjacent exterior angle, so

and

.

The sum of the degree measures of the three interior angles is 180, so

Example Question #2 : How To Find An Angle In An Acute / Obtuse Triangle

Triangle 3

In the above figure, .

Give the measure of .

Possible Answers:

Correct answer:

Explanation:

 and  form a linear pair, so their degree measures total ; consequently,

 

, so by the Isosceles Triangle Theorem, 

 

The sum of the degree measures of a triangle is , so 

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

Triangle

Figure NOT drawn to scale.

Refer to the above figure. Evaluate .

Possible Answers:

Correct answer:

Explanation:

The measure of an exterior angle of a triangle, which here is , is equal to the sum of the measures of its remote interior angles, which here are  and . Consequently,

 and  form a linear pair and, therefore,

.

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