Acute / Obtuse Isosceles Triangles

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ACT Math › Acute / Obtuse Isosceles Triangles

Questions 1 - 10
1

What is the height of an isosceles triangle which has a base of and an area of ?

Explanation

The area of a triangle is given by the equation:

In this case, we are given the area () and the base () and are asked to solve for height ().

To do this, we must plug in the given values for and , which gives the following:

We then must multiply the right side, and then divide the entire equation by 2, in order to solve for :

Therefore, the height of the triangle is .

2

What is the height of an isosceles triangle which has a base of and an area of ?

Explanation

The area of a triangle is given by the equation:

In this case, we are given the area () and the base () and are asked to solve for height ().

To do this, we must plug in the given values for and , which gives the following:

We then must multiply the right side, and then divide the entire equation by 2, in order to solve for :

Therefore, the height of the triangle is .

3

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?

Explanation

Every triangle has . An isosceles triangle has one vertex ange, and two congruent base angles.

Let be the vertex angle and be the base angle.

The equation to solve becomes , since the base angle occurs twice.

Now we can solve for the vertex angle.

The difference between the vertex angle and the base angle is .

4

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?

Explanation

Every triangle has . An isosceles triangle has one vertex ange, and two congruent base angles.

Let be the vertex angle and be the base angle.

The equation to solve becomes , since the base angle occurs twice.

Now we can solve for the vertex angle.

The difference between the vertex angle and the base angle is .

5

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

7\ cm

6\ cm

8\ cm

9\ cm

10\ cm

Explanation

A=\frac{1}{2}bh.

6x=42

x=7

6

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

7\ cm

6\ cm

8\ cm

9\ cm

10\ cm

Explanation

A=\frac{1}{2}bh.

6x=42

x=7

7

In the standard coordinate plane, the points and form two vertices of an isosceles triangle. Which of the following points could be the third vertex?

Explanation

To form an isosceles triangle here, we need to create a third vertex whose coordinate is between and . If a vertex is placed at , the distance from to this point will be . The distance from to this point will be the same.

8

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

140 degrees

70 degrees

40 degrees

100 degrees

None of the other answers

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.

9

In the standard coordinate plane, the points and form two vertices of an isosceles triangle. Which of the following points could be the third vertex?

Explanation

To form an isosceles triangle here, we need to create a third vertex whose coordinate is between and . If a vertex is placed at , the distance from to this point will be . The distance from to this point will be the same.

10

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

140 degrees

70 degrees

40 degrees

100 degrees

None of the other answers

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.

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