High School Math : How to find an angle in an acute / obtuse isosceles triangle

Study concepts, example questions & explanations for High School Math

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Example Questions

Example Question #11 : Isosceles Triangles

An isoceles triangle has a base angle that is \(\displaystyle 15\) degrees less than three times the vertex angle.  What is the product of the vertex angle and the base angle?

Possible Answers:

\(\displaystyle 3375\)

\(\displaystyle 3750\)

\(\displaystyle 4500\)

\(\displaystyle 2250\)

\(\displaystyle 1875\)

Correct answer:

\(\displaystyle 2250\)

Explanation:

Every triangle has 180 degrees.  An isoceles triangle has one vertex angle and two congruent base angles.

Let \(\displaystyle x=\) vertex angle and \(\displaystyle 3x-15=\) base angle.

Then the equation to solve becomes:

\(\displaystyle x+(3x-15)+(3x-15)=180\), or \(\displaystyle 7x-30=180\).

Then the vertex angle is \(\displaystyle 30\), the base angle is \(\displaystyle 75\), and the product is \(\displaystyle 2250\).

 

 

Example Question #224 : Geometry

In triangle ABC, Angle A = x degrees, Angle B = 2x degrees, and Angle C = 3x+30 degrees. How many degrees is Angle B?

Possible Answers:

45°

25°

50°

30°

105°

Correct answer:

50°

Explanation:

Because the interior angles of a triangle add up to 180°, we can create an equation using the variables given in the problem: x+2x+(3x+30)=180. This simplifies to 6X+30=180. When we subtract 30 from both sides, we get 6x=150. Then, when we divide both sides by 6, we get x=25. Because Angle B=2x degrees, we multiply 25 times 2. Thus, Angle B is equal to 50°. If you got an answer of 25, you may have forgotten to multiply by 2. If you got 105, you may have found Angle C instead of Angle B.

Example Question #221 : Geometry

An isosceles triangle has a base angle that is six more than three times the vertex angle.  What is the base angle?

Possible Answers:

\(\displaystyle 35\)

\(\displaystyle 78\)

\(\displaystyle 24\)

\(\displaystyle 66\)

\(\displaystyle 82\)

Correct answer:

\(\displaystyle 78\)

Explanation:

Every triangle has 180 degrees.  An isosceles triangle has one vertex angle and two congruent base angles.

Let \(\displaystyle x\) = vertex angle and \(\displaystyle 3x + 6\) = base angle.

Then the equation to solve becomes

 \(\displaystyle x + (3x + 6) + (3x + 6) = 180\)

or

\(\displaystyle 7x + 12 = 180\).

Solving for \(\displaystyle x\) gives a vertex angle of 24 degrees and a base angle of 78 degrees.

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

The base angle of an isosceles triangle is thirteen more than three times the vertex angle. What is the difference between the vertex angle and the base angle?

Possible Answers:

\(\displaystyle 79^o\)

\(\displaystyle 22^o\)

\(\displaystyle 101^o\)

\(\displaystyle 57^o\)

\(\displaystyle 65^o\)

Correct answer:

\(\displaystyle 57^o\)

Explanation:

Every triangle has \(\displaystyle 180^o\). An isosceles triangle has one vertex ange, and two congruent base angles.

Let \(\displaystyle x\) be the vertex angle and \(\displaystyle 3x + 13\) be the base angle.

The equation to solve becomes \(\displaystyle x + (3x + 13) + (3x + 13) = 180^o\), since the base angle occurs twice.

\(\displaystyle 7x + 26 = 180^o\)

\(\displaystyle 7x=154^o\)

\(\displaystyle x=22^o\)

Now we can solve for the vertex angle.

\(\displaystyle 3x+13=3(22)+13=79^o\)

The difference between the vertex angle and the base angle is \(\displaystyle 79^o - 22^o = 57^o\).

Example Question #11 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

An isoceles triangle has a base angle that is five less than twice the vertex angle.  What is the sum of the base and vertex angles?

Possible Answers:

\(\displaystyle 33\)

\(\displaystyle 71\)

\(\displaystyle 38\)

\(\displaystyle 109\)

\(\displaystyle 135\)

Correct answer:

\(\displaystyle 109\)

Explanation:

Each triangle has \(\displaystyle 180\) degrees.

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

Let \(\displaystyle x=\) vertex angle and \(\displaystyle 2x-5=\) base angle.

Then the equation to solve becomes \(\displaystyle x+(2x-5)+(2x-5)=180\) or \(\displaystyle 5x-10=180\).

Add \(\displaystyle 10\) to both sides to get \(\displaystyle 5x=190\).

Divide both sides by \(\displaystyle 5\) to get \(\displaystyle x=38\) vertex angle and \(\displaystyle 2x-5=71\) base angles, so the sum of the angles is \(\displaystyle 109\).

Example Question #281 : Plane Geometry

An isoceles triangle has a base angle that is twice the vertex angle.  What is the sum of one base angle and the vertex angle?

Possible Answers:

\(\displaystyle 108\)

\(\displaystyle 36\)

\(\displaystyle 135\)

\(\displaystyle 85\)

\(\displaystyle 72\)

Correct answer:

\(\displaystyle 108\)

Explanation:

Every triangle contains \(\displaystyle 180\) degrees.  An isoceles triangle has two congruent base angles and one vertex angle.

Let \(\displaystyle x=\) the vertex angle and \(\displaystyle 2x =\) the base angle

So the equation to solve becomes \(\displaystyle x+2x+2x=180\) or \(\displaystyle 5x=180\) and dividing by \(\displaystyle 5\) gives \(\displaystyle x = 36\) for the vertex angle and \(\displaystyle 2x = 72\) for the base angle, so the sum is \(\displaystyle 108\)

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