High School Math : Acute / Obtuse Isosceles Triangles

Study concepts, example questions & explanations for High School Math

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Example Questions

Example Question #1 : Acute / Obtuse Isosceles Triangles

An isosceles triangle has a base of 12\ cm\(\displaystyle 12\ cm\) and an area of 42\ cm^{2}\(\displaystyle 42\ cm^{2}\). What must be the height of this triangle?

Possible Answers:

8\ cm\(\displaystyle 8\ cm\)

6\ cm\(\displaystyle 6\ cm\)

7\ cm\(\displaystyle 7\ cm\)

9\ cm\(\displaystyle 9\ cm\)

10\ cm\(\displaystyle 10\ cm\)

Correct answer:

7\ cm\(\displaystyle 7\ cm\)

Explanation:

A=\frac{1}{2}bh\(\displaystyle A=\frac{1}{2}bh\)

6x=42\(\displaystyle 6x=42\)

x=7\(\displaystyle x=7\)

Example Question #1 : How To Find The Perimeter Of An Acute / Obtuse Isosceles Triangle

One side of an acute isosceles triangle is 15 feet.  Another side is 5 feet.  What is the perimeter of the triangle in feet?

Possible Answers:

\(\displaystyle 20\)

\(\displaystyle 35\)

\(\displaystyle 30\)

\(\displaystyle 45\)

\(\displaystyle 75\)

Correct answer:

\(\displaystyle 35\)

Explanation:

Because this is an acute isosceles triangle, the third side must be the same as the longer of the sides that you were given.  To find the perimeter, multiply the longer side by 2 and add the shorter side.

\(\displaystyle (15\cdot2)+5=35\)

Example Question #3 : Isosceles Triangles

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

Possible Answers:

\(\displaystyle 107^{\circ}\)

\(\displaystyle 68^{\circ}\)

\(\displaystyle 39^{\circ}\)

\(\displaystyle 34^{\circ}\)

\(\displaystyle 73^{\circ}\)

Correct answer:

\(\displaystyle 39^{\circ}\)

Explanation:

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

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

So the equation to solve becomes

\(\displaystyle x+2x+5+2x+5 = 180\) or \(\displaystyle 5x+10=180\)

So the vertex angle is \(\displaystyle 34^{\circ}\) and the base angle is \(\displaystyle 73^{\circ}\) so the difference is \(\displaystyle 39^{\circ}\)

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

Points A and B lie on a circle centered at Z, where central angle <AZB measures 140°. What is the measure of angle <ZAB?

 

Possible Answers:

25°

Cannot be determined from the given information

15°

20°

30°

Correct answer:

20°

Explanation:

Because line segments ZA and ZB are radii of the circle, they must have the same length. That makes triangle ABZ an isosceles triangle, with <ZAB and <ZBA having the same measure. Because the three angles of a triangle must sum to 180°, you can express this in the equation:

140 + 2x = 180 --> 2x = 40 --> x = 20

 

 

 

Example Question #221 : Geometry

Triangle FGH has equal lengths for FG and GH; what is the measure of F, if G measures 40 degrees? 

Possible Answers:

100 degrees

None of the other answers

70 degrees

140 degrees

40 degrees

Correct answer:

70 degrees

Explanation:

It's good to draw a diagram for this; we know that it's an isosceles triangle; remember that the angles of a triangle total 180 degrees.

Angle G for this triangle is the one angle that doesn't correspond to an equal side of the isosceles triangle (opposite side to the angle), so that means F = H, and that F + H + 40 = 180,

By substitution we find that F * 2 = 140 and angle F = 70 degrees. 

Example Question #222 : Geometry

The vertex angle of an isosceles triangle is \(\displaystyle 54^{\circ}\).  What is the base angle?

Possible Answers:

\(\displaystyle 47^{\circ}\)

\(\displaystyle 36^{\circ}\)

\(\displaystyle 54^{\circ}\)

\(\displaystyle 72^{\circ}\)

\(\displaystyle 63^{\circ}\)

Correct answer:

\(\displaystyle 63^{\circ}\)

Explanation:

An isosceles triangle has two congruent base angles and one vertex angle.  Each triangle contains \(\displaystyle 180^{\circ}\).  Let \(\displaystyle x\) = base angle, so the equation becomes \(\displaystyle 54 + x + x = 180\).  Solving for \(\displaystyle x\) gives \(\displaystyle x = 63\)

Example Question #221 : Act Math

In an isosceles triangle the base angle is five less than twice the vertex angle.  What is the sum of the vertex angle and the base angle?

Possible Answers:

\(\displaystyle 154\)

\(\displaystyle 76\)

\(\displaystyle 33\)

\(\displaystyle 142\)

\(\displaystyle 109\)

Correct answer:

\(\displaystyle 109\)

Explanation:

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

Let \(\displaystyle x\) = the vertex angle

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

So the equation to solve becomes 

\(\displaystyle x + (2x - 5) + (2x - 5) = 180\)

or

\(\displaystyle 5x - 10 = 180\)

Thus the vertex angle is 38 and the base angle is 71 and their sum is 109.

Example Question #223 : Geometry

Sides \(\displaystyle AB\) and \(\displaystyle AC\) in this triangle are equal. What is the measure of \(\displaystyle \angle A\)?

Triangle_1

Possible Answers:

\(\displaystyle 50^{\circ}\)

\(\displaystyle 40^{\circ}\)

\(\displaystyle 65^{\circ}\)

\(\displaystyle 130^{\circ}\)

\(\displaystyle 180^{\circ}\)

Correct answer:

\(\displaystyle 50^{\circ}\)

Explanation:

This triangle has an angle of \(\displaystyle 65^{\circ}\). We also know it has another angle of \(\displaystyle 65^{\circ}\) at \(\displaystyle \angle ABC\) because the two sides are equal. Adding those two angles together gives us \(\displaystyle 130^{\circ}\) total. Since a triangle has \(\displaystyle 180^{\circ}\) total, we subtract 130 from 180 and get 50.

Example Question #1 : Isosceles Triangles

An isosceles triangle has a vertex angle that is twenty degrees more than twice the base angle.  What is the sum of the vertex and base angles?

Possible Answers:

40

\(\displaystyle 140\)

\(\displaystyle 75\)

\(\displaystyle 120\)

\(\displaystyle 60\)

Correct answer:

\(\displaystyle 140\)

Explanation:

All triangles contain \(\displaystyle 180\) degrees.  An isosceles triangle has one vertex angle and two congruent base angles.

Let \(\displaystyle x = base\) and \(\displaystyle 2x+20 = vertex\).

So the equation to solve becomes \(\displaystyle 2x+20+x+x=180\).

We get \(\displaystyle x=40\) and \(\displaystyle 2x+20 = 100\), so the sum of the base and vertex angles is \(\displaystyle 140\).

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

If an isosceles triangle has an angle measuring greater than 100 degrees, and another angle with a measuring \(\displaystyle z\) degrees, which of the following is true?

Possible Answers:

\(\displaystyle z = 40 \: degrees\)

\(\displaystyle z < 40 \: degrees\)

\(\displaystyle z < 80 \: degrees\)

\(\displaystyle z = 60 \: degrees\)

\(\displaystyle z < 100\: degrees\)

Correct answer:

\(\displaystyle z < 40 \: degrees\)

Explanation:

In order for a triangle to be an isosceles triangle, it must contain two equivalent angles and one angle that is different. Given that one angle is greater than 100 degrees: \(\displaystyle 180 - 100 = 80 \: degrees.\) Thus, the sum of the other two angles must be less than 80 degrees. If an angle is represented by \(\displaystyle z\): \(\displaystyle 2z < 80\: degrees = z < 40\: degrees.\)

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