GMAT Math : Calculating an angle in an acute / obtuse triangle

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Example Question #3 : Acute / Obtuse Triangles

Which of the following cannot be the measure of a base angle of an isosceles triangle?

Possible Answers:

$100 ^{\circ }$

$1 ^{\circ }$

Each of the other choices can be the measure of a base angle of an isosceles triangle.

$50^{\circ }$

$80 ^{\circ }$

Correct answer:

$100 ^{\circ }$

Explanation:

An isosceles triangle has two congruent angles by the Isosceles Triangle Theorem; these are the base angles. Since at least two angles of any triangle must be acute, a base angle must be acute - that is, it must measure under $90^{\circ }$ . The only choice that does not fit this criterion is $100 ^{\circ }$ , making this the correct choice.

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Example Question #4 : Acute / Obtuse Triangles

Let the three interior angles of a triangle measure x, x+30 , and 2x - 10 . Which of the following statements is true about the triangle?

Possible Answers:

The triangle is isosceles and acute.

The triangle is isosceles and obtuse.

The triangle is scalene and right.

The triangle is scalene and obtuse.

The triangle is scalene and acute.

Correct answer:

The triangle is isosceles and acute.

Explanation:

If these are the measures of the interior angles of a triangle, then they total $180^{\circ }$ . Add the expressions, and solve for .

$x +\left ( x + 30 \right ) +\left ( 2x - 10 \right ) = 180$

4x +20 = 180

4x = 160

x = 40

One angle measures $40^{\circ }$ . The others measure:

$x + 30 = 40 + 30 = 70^{\circ }$

$2x-10 = 2 \cdot 40 - 10 = 80 - 10 = 70 ^{\circ }$

All three angles measure less than $90 ^{\circ }$ , so the triangle is acute. Also, there are two congruent angles, so by the converse of the Isosceles Triangle Theorem, two sides are congruent, and the triangle is isosceles.

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Example Question #1 : Calculating An Angle In An Acute / Obtuse Triangle

Two angles of an isosceles triangle measure $(x + 20) ^{\circ}$ and $\left ( 100-x\right )^{ \circ}$ . What are the possible values of ?

Possible Answers:

$x = 40 \textrm{ or }x = 80$

x = 40

x = 60

x = 80

$x = 60 \textrm{ or } x = 80$

Correct answer:

x = 40

Explanation:

In an isosceles triangle, at least two angles measure the same. Therefore, one of three things happens:

Case 1: The two given angles have the same measure.

x + 20 = 100 - x

x+ 20 + x = 100 - x + x

2x+ 20 = 100

2x + 20 - 20= 100 -20

2x = 80

$2x \div 2 =80 \div 2$

x = 40

The angle measures are $60 ^{\circ}, 60 ^{\circ}, 60 ^{\circ}$ , making the triangle equianglular and, subsequently, equilateral. An equilateral triangle is considered isosceles, so this is a possible scenario.

Case 2: The third angle has measure $(x + 20) ^{\circ}$ .

Then, since the sum of the angle measures is 180,

$\left (x + 20 \right ) +\left (x +20 \right )+ \left ( 100-x \right ) = 180$

x + 140= 180

x + 140 - 140 = 180- 140

x = 40

as before

Case 3: The third angle has measure $\left ( 100-x\right )^{ \circ}$

$\left (x + 20 \right ) +\left (100-x \right )+ \left ( 100-x \right ) = 180$

220 -x = 180

220 -x -220 = 180-220

-x = -40

x = 40

as before.

Thus, the only possible value of is 40.

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Example Question #2 : Calculating An Angle In An Acute / Obtuse Triangle

Two angles of an isosceles triangle measure $\left (x + 30 \right )^{\circ }$ and $\left (x + 36 \right )^{\circ }$ . What are the possible value(s) of ?

Possible Answers:

x= 12

x=6

$x=26 \textrm{ or }x=28$

$x= 12, \; x=26 \textrm{, or }x=28$

$x= 6, \; x=26 \textrm{, or }x=28$

Correct answer:

$x=26 \textrm{ or }x=28$

Explanation:

In an isosceles triangle, at least two angles measure the same. Therefore, one of three things happens:

Case 1: The two given angles have the same measure.

x + 30 = x + 36

x + 30-x = x + 36-x

30=36

This is a false statement, indicating that this situation is impossible.

Case 2: The third angle has measure $\left (x + 30 \right )^{\circ }$ .

Then, since the sum of the angle measures is 180,

$\left (x + 30 \right ) +\left (x + 30 \right )+ \left ( x + 36 \right ) = 180$

3x + 96 = 180

3x =84

x=28

This makes the angle measures $58^{\circ },58^{\circ },64^{\circ }$ , a plausible scenario.

Case 3: the third angle has measure $\left (x + 36 \right )^{\circ }$

Then, since the sum of the angle measures is 180,

$\left (x + 30 \right ) +\left (x + 36 \right )+ \left ( x + 36 \right ) = 180$

3x +102 = 180

3x =78

x=26

This makes the angle measures $56^{\circ }, 62^{\circ }, 62^{\circ }$ , a plausible scenario.

Therefore, either x=26 or x=28

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Example Question #2 : Calculating An Angle In An Acute / Obtuse Triangle

$\Delta ABC \sim \Delta XYZ$

$m \angle A = 20 ^{\circ }; m \angle Y = 120 ^{\circ }$

Which of the following is true of $\Delta ABC$ ?

Possible Answers:

$\Delta ABC$ may be scalene or isosceles, but it is acute,

$\Delta ABC$ is isosceles and obtuse.

$\Delta ABC$ is scalene and acute.

$\Delta ABC$ may be scalene or isosceles, but it is obtuse.

$\Delta ABC$ is scalene and obtuse.

Correct answer:

$\Delta ABC$ is scalene and obtuse.

Explanation:

By similarity, $m \angle B = m \angle Y = 120 ^{\circ }$ .

Since measures of the interior angles of a triangle total $180 ^{\circ }$ ,

$m \angle A + m \angle B + m \angle C = 180$

$20 + 120 + m \angle C = 180$

$140 + m \angle C = 180$

$140 + m \angle C -140 = 180-140$

$m \angle C =40^{\circ }$

Since the three angle measures of $\Delta ABC$ are all different, no two sides measure the same; the triangle is scalene. Also, since $m \angle B = 120 ^{\circ } > 90^{\circ }$ , the angle is obtuse, and $\Delta ABC$ is an obtuse triangle.

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Example Question #1 : Calculating An Angle In An Acute / Obtuse Triangle

Which of the following is true of a triangle with three angles whose measures have an arithmetic mean of $90^{\circ }$ ?

Possible Answers:

The triangle may be right or obtuse but must be scalene.

The triangle cannot exist.

The triangle must be right and isosceles.

The triangle must be obtuse but may be scalene or isosceles.

The triangle must be right but may be scalene or isosceles.

Correct answer:

The triangle cannot exist.

Explanation:

The sum of the measures of three angles of any triangle is 180; therefore, their mean is $180 \div 3 = 60$ , making a triangle with angles whose measures have mean 90 impossible.

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Example Question #2 : Calculating An Angle In An Acute / Obtuse Triangle

Two angles of a triangle measure $47 ^{\circ }$ and $23^{\circ }$ . What is the measure of the third angle?

Possible Answers:

$70^{\circ }$

$20^{\circ }$

$160^{\circ }$

$35^{ \circ }$

$110^{\circ }$

Correct answer:

$110^{\circ }$

Explanation:

The sum of the degree measures of the angles of a triangle is 180, so we can subtract the two angle measures from 180 to get the third:

$(180-47-23)^{\circ } = 110^{\circ }$

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Example Question #332 : Geometry

The angles of a triangle measure $x^{\circ }, x^{\circ },\left ( x -54 \right ) ^{\circ }$ . Evaluate .

Possible Answers:

x = 96

x = 112

x = 78

x = 84

x = 72

Correct answer:

x = 78

Explanation:

The sum of the measures of the angles of a triangle total $180^{\circ }$ , so we can set up and solve for in the following equation:

x + x + (x-54) = 180

3x-54 = 180

3x-54 + 54 = 180+ 54

3x =234

$3x\div 3 =234 \div 3$

x = 78

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Example Question #7 : Calculating An Angle In An Acute / Obtuse Triangle

An exterior angle of $\Delta ABC$ with vertex measures $150^{\circ }$ ; an exterior angle of $\Delta ABC$ with vertex measures $110^{\circ }$ . Which is the following is true of $\Delta ABC$ ?

Possible Answers:

$\Delta ABC$ is obtuse and isosceles

$\Delta ABC$ is acute and scalene

$\Delta ABC$ is right and scalene

$\Delta ABC$ is obtuse and scalene

$\Delta ABC$ is acute and isosceles

Correct answer:

$\Delta ABC$ is acute and scalene

Explanation:

An interior angle of a triangle measures $180 ^{\circ }$ minus the degree measure of its exterior angle. Therefore:

$m \angle A = 180 ^{\circ } - 150 ^{\circ } = 30 ^{\circ }$

$m \angle B= 180 ^{\circ } - 110 ^{\circ } = 70 ^{\circ }$

The sum of the degree measures of the interior angles of a triangle is $180 ^{\circ }$ , so

$m \angle C = 180 ^{\circ } - (m \angle A + m \angle B) = 180 ^{\circ } - (30 ^{\circ }+ 70 ^{\circ }) = 180 ^{\circ } -100 ^{\circ } = 80 ^{\circ }$ .

Each angle is acute, so the triangle is acute; each angle is of a different measure, so the triangle has three sides of different measure, making it scalene.

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Example Question #2 : Calculating An Angle In An Acute / Obtuse Triangle

Lines

Note: Figure NOT drawn to scale.

Refer to the above diagram.

$m\angle DCF = 140 ^{\circ } , m \angle ABF = 134 ^{\circ }$

Evaluate $m \angle EFB$ .

Possible Answers:

$84^{\circ }$

$96^{\circ }$

$86^{\circ }$

$74^{\circ }$

$76^{\circ }$

Correct answer:

$86^{\circ }$

Explanation:

The sum of the exterior angles of a triangle, one per vertex, is $360 ^{\circ}$ . $\angle DCF$ , $\angle ABF$ and $\angle EFB$ are exterior angles at different vertices, so

$m \angle EFB + m\angle DCF + m \angle ABF = 360 ^{\circ }$

$m \angle EFB + 140 ^{\circ } + 134 ^{\circ } = 360 ^{\circ }$

$m \angle EFB + 274 ^{\circ } = 360 ^{\circ }$

$m \angle EFB + 274 ^{\circ }- 274 ^{\circ } = 360 ^{\circ } - 274 ^{\circ }$

$m \angle EFB = 86^{\circ }$

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GMAT Math : Calculating an angle in an acute / obtuse triangle

Study concepts, example questions & explanations for GMAT Math

All GMAT Math Resources

Example Questions

Example Question #3 : Acute / Obtuse Triangles

Example Question #4 : Acute / Obtuse Triangles

Example Question #1 : Calculating An Angle In An Acute / Obtuse Triangle

Example Question #2 : Calculating An Angle In An Acute / Obtuse Triangle

Example Question #2 : Calculating An Angle In An Acute / Obtuse Triangle

Example Question #1 : Calculating An Angle In An Acute / Obtuse Triangle

Example Question #2 : Calculating An Angle In An Acute / Obtuse Triangle

Example Question #332 : Geometry

Example Question #7 : Calculating An Angle In An Acute / Obtuse Triangle

Example Question #2 : Calculating An Angle In An Acute / Obtuse Triangle

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