Angle facts and parallel lines

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What is an angle?

An angle is formed when two lines meet at a point.
Each line is called a side of the angle; the point they share is the vertex.
The size of an angle is measured in degrees (°).
A full turn is 360°, so a half turn is 180° and a quarter turn is 90°.
Angles can be named using three letters with the vertex in the middle, e.g. angle BAC, or just by the vertex letter, e.g. angle A.

Types of angle

Acute: less than 90°.
Right: exactly 90°.
Obtuse: between 90° and 180°.
Straight: exactly 180°.
Reflex: between 180° and 360°.
Zero: 0° (not turned).

Measuring and drawing angles

Use a protractor to measure angles accurately.
Place the centre of the protractor on the vertex and line up the 0° mark with one side.
Read the scale that starts from 0° on the side you lined up.
To draw an angle, draw one side, place the protractor correctly, mark the required degree, then draw the second side.
Estimate first: acute angles are less than 90°, obtuse between 90° and 180°, reflex greater than 180°.

Angle facts: straight lines and points

Angles on a straight line add up to 180°.
Angles around a point add up to 360°.
Vertically opposite angles are equal.
These facts let you find missing angles without measuring.

Angles in triangles and quadrilaterals

The angles in a triangle add up to 180°.
The angles in a quadrilateral add up to 360°.
In an isosceles triangle, the angles opposite the equal sides are equal.
In an equilateral triangle, all angles are 60°.

Parallel lines and transversals

Parallel lines are always the same distance apart and never meet.
A transversal is a line that crosses two or more lines.
When a transversal crosses parallel lines, it creates equal and supplementary angle pairs.
Arrow markings (▸▸) show lines are parallel.

Alternate, corresponding and co-interior angles

Alternate angles are equal. They are on opposite sides of the transversal and between the parallel lines (forming a Z shape).
Corresponding angles are equal. They are in the same position at each intersection (forming an F shape).
Co-interior angles add up to 180°. They are on the same side of the transversal and between the parallel lines (forming a C or U shape).
These angle facts are true only when the lines are parallel.

Giving reasons

For each step in an angle calculation, write a reason.
Common reasons: angles on a straight line sum to 180°; angles around a point sum to 360°; vertically opposite angles are equal; angles in a triangle sum to 180°; alternate angles are equal; corresponding angles are equal; co-interior angles sum to 180°.
Use the correct notation: angle ABC or ∠ABC, and the degree symbol °.
Show all steps clearly so your method can be followed.

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Câu hỏi luyện tập

Xem trước miễn phí — 8 trên 62 câu hỏi. Đăng ký để xem tất cả.

1.Which type of angle measures exactly 90°?
Easy
- AAcute
- BRight
- CObtuse
- DStraight
2.An angle that is greater than 90° but less than 180° is called:
Easy
- AAcute
- BRight
- CObtuse
- DReflex
3.What is the sum of angles on a straight line?
Easy
- A90°
- B180°
- C270°
- D360°
4.What is the sum of angles around a point?
Easy
- A90°
- B180°
- C270°
- D360°
5.In a triangle, the angles are 50° and 60°. What is the third angle?
Easy
- A60°
- B70°
- C80°
- D90°
6.Two angles are vertically opposite. If one angle is 40°, what is the other?
Easy
- A40°
- B50°
- C140°
- D180°
7.A quadrilateral has angles 80°, 100°, and 90°. What is the fourth angle?
Medium
- A80°
- B90°
- C100°
- D110°
8.Two parallel lines are cut by a transversal. Which pair of angles are equal?
Medium
- ACo-interior angles
- BAlternate angles
- CSupplementary angles
- DAdjacent angles on a straight line

Thêm câu hỏi

9.In a triangle, two angles are equal and the third angle is 80°. What is the size of each equal angle?
Medium
- A40°
- B50°
- C60°
- D70°
10.Angles on a straight line are 3x, 2x, and 40°. What is the value of x?
Medium
- A20
- B28
- C30
- D40
11.A regular hexagon has interior angles of 120° each. What is the sum of its interior angles?
Medium
- A540°
- B720°
- C900°
- D1080°
12.Two parallel lines are cut by a transversal. One co-interior angle is 110°. What is the other co-interior angle?
Medium
- A70°
- B80°
- C110°
- D180°
13.Which of the following statements are true about angles? (select all that apply)
Medium
- AAngles on a straight line sum to 180°.
- BVertically opposite angles are equal.
- CAlternate angles are equal when lines are parallel.
- DCo-interior angles are equal when lines are parallel.
- EThe sum of angles in a triangle is 360°.
14.Which of the following are units used to measure angles? (select all that apply)
Medium
- ADegree
- BRadian
- CTurn
- DMetre
- ENewton
15.An acute angle is less than 90°.
Easy
True or false?
16.A reflex angle is between 180° and 360°.
Easy
True or false?
17.When two parallel lines are cut by a transversal, corresponding angles are equal.
Easy
True or false?
18.The sum of angles in any quadrilateral is 360°.
Easy
True or false?
19.Match each angle type with its description.
Medium
- Acute
- Right
- Obtuse
- Reflex
- Less than 90°
- Exactly 90°
- Between 90° and 180°
- Between 180° and 360°
20.Place these angle types in order from smallest to largest.
Medium
- Acute
- Right
- Obtuse
- Straight
- Reflex
21.Two parallel lines are cut by a transversal. Select all statements that are always true.
Medium
- AAlternate angles are equal.
- BCorresponding angles are equal.
- CCo-interior angles add to 180°.
- DVertically opposite angles add to 180°.
- ECo-interior angles are equal.
22.Match each angle fact with the correct statement.
Medium
- Angles on a straight line
- Angles around a point
- Vertically opposite angles
- Angles in a triangle
- sum to 180°
- sum to 360°
- are equal
23.Order these angle sizes from smallest to largest.
Medium
- 40°
- 90°
- 150°
- 210°
24.Which of the following angle pairs are equal when two parallel lines are cut by a transversal? (Select all that apply.)
Medium
- AAlternate angles
- BCorresponding angles
- CCo-interior angles
- DVertically opposite angles
25.Arrange the steps to find the size of an unknown angle x on a straight line where the other angle is 65°.
Medium
- Write the equation x + 65 = 180.
- Subtract 65 from both sides.
- State that angles on a straight line sum to 180°.
- Conclude x = 115°.
26.In a triangle, the angles are (2x)°, (3x)° and 40°. What is the value of x?
Hard
- A28
- B30
- C35
- D40
27.Which of the following statements are true about angles in a quadrilateral? (Select all that apply.)
Medium
- AThe sum of interior angles is 360°.
- BOpposite angles are always equal.
- CThe sum of exterior angles is 360°.
- DAll angles are always 90°.
28.Match each angle type with its size description.
Medium
- Acute
- Right
- Obtuse
- Reflex
- less than 90°
- exactly 90°
- between 90° and 180°
- between 180° and 360°
29.In a triangle, two angles are equal and the third angle is 40°. Which of the following are true? (Select all that apply.)
Hard
- AEach equal angle is 70°.
- BThe sum of the two equal angles is 140°.
- CThe triangle is isosceles.
- DEach equal angle is 80°.
30.Two parallel lines are cut by a transversal. One angle is 120°. Which of the following angle sizes also appear in the diagram? (Select all that apply.)
Hard
- A60°
- B120°
- C240°
- D90°
31.Order these steps to find the size of angle y in a triangle where the other two angles are 50° and 60°.
Hard
- Subtract 110 from 180 to get y = 70°.
- Add 50 and 60 to get 110°.
- State that angles in a triangle sum to 180°.
- Write the equation y + 50 + 60 = 180.
32.Which of the following are valid reasons for angle calculations? (Select all that apply.)
Hard
- AAngles on a straight line sum to 180°.
- BAngles around a point sum to 360°.
- CVertically opposite angles are equal.
- DAlternate angles are equal.
- ECo-interior angles are equal.
33.Two angles lie on a straight line. One of them is 115°. What is the size of the other angle?
Easy
- A55°
- B65°
- C75°
- D245°
34.Three angles meet at a point. Two of them are 130° and 95°. What is the size of the third angle?
Easy
- A125°
- B135°
- C145°
- D225°
35.Two straight lines cross. One of the four angles is 62°. What is the size of the angle vertically opposite it?
Medium
- A28°
- B62°
- C118°
- D298°
36.A triangle has angles of 37° and 68°. What is the size of the third angle?
Medium
- A65°
- B75°
- C85°
- D105°
37.A quadrilateral has angles of 95°, 85° and 110°. What is the size of the fourth angle?
Medium
- A60°
- B70°
- C80°
- D90°
38.Two parallel lines are cut by a transversal. One corresponding angle is 73°. What is the size of the other corresponding angle in that pair?
Medium
- A17°
- B73°
- C107°
- D287°
39.Two parallel lines are cut by a transversal. One alternate angle is 48°. What is the size of the other alternate angle in that pair?
Medium
- A42°
- B48°
- C132°
- D312°
40.Two parallel lines are cut by a transversal. One co-interior angle is 124°. What is the size of the other co-interior angle?
Medium
- A56°
- B66°
- C124°
- D236°
41.Name the type of angle that is exactly 180°.
Easy
42.What is the term for angles that are opposite each other when two lines intersect?
Easy
43.Two angles on a straight line are 70° and x. Find x.
Medium
44.In a triangle, two angles are 45° and 55°. Find the third angle.
Medium
45.Angles around a point are 120°, 90°, and 80°. Find the fourth angle.
Medium
46.Complete the sentence about angle types.
Easy
An angle less than 90° is called an ____ angle.
47.Complete the sentence about parallel lines.
Easy
When two parallel lines are cut by a transversal, ____ angles are equal.
48.Name the three angle facts.
Medium
Angles on a straight line sum to ____, angles around a point sum to ____, and angles in a triangle sum to ____.
49.Two angles on a straight line are 83° and x. Find the value of x, in degrees.
Easy
50.In a triangle, two angles are 41° and 59°. Find the size of the third angle, in degrees.
Medium
51.Two parallel lines are cut by a transversal. One co-interior angle is 118°. Find the size of the other co-interior angle, in degrees.
Medium
52.Three angles meet at a point and are 2x, 3x and 4x. Find the value of x.
Medium
53.Explain why the sum of angles in a triangle is 180°.
Medium
54.Explain why the angles in any triangle add to 180°.
Hard
55.Two parallel lines are cut by a transversal. Explain why co-interior angles add to 180°.
Medium
56.Two angles on a straight line are 70° and x. Find x.
Medium
- A121 °
- B99 °
- C132 °
- D110 °
57.In a triangle, two angles are 45° and 55°. Find the third angle.
Medium
- A88 °
- B96 °
- C72 °
- D80 °
58.Angles around a point are 120°, 90°, and 80°. Find the fourth angle.
Medium
- A84 °
- B70 °
- C77 °
- D63 °
59.Two angles on a straight line are 83° and x. Find the value of x, in degrees.
Easy
- A107 °
- B117 °
- C97 °
- D87 °
60.In a triangle, two angles are 41° and 59°. Find the size of the third angle, in degrees.
Medium
- A72 °
- B80 °
- C88 °
- D96 °
61.Two parallel lines are cut by a transversal. One co-interior angle is 118°. Find the size of the other co-interior angle, in degrees.
Medium
- A62 °
- B68 °
- C74 °
- D56 °
62.Three angles meet at a point and are 2x, 3x and 4x. Find the value of x.
Medium
- A44
- B48
- C40
- D36

Ghi chú ôn tập

## Angle Facts and Parallel Lines ### What is an angle? An angle is formed by two lines that meet at a point. Each line is a **side** of the angle; the shared point is the **vertex**. The size of an angle is its **measure**. Notation: \(\angle A\) (vertex alone) or \(\angle BAC\) (vertex in the middle). The measure is written \(m\angle A\). ### Types of angle - **Zero**: 0° - **Acute**: less than 90° - **Right**: exactly 90° (lines are perpendicular) - **Obtuse**: between 90° and 180° - **Straight**: exactly 180° - **Reflex**: between 180° and 360° A full turn is 360°. ### Angle facts - **Straight line**: angles add to 180°. - **Around a point**: angles add to 360°. - **Vertically opposite**: equal. - **Triangle**: angles add to 180°. - **Quadrilateral**: angles add to 360°. ### Parallel lines A transversal crossing parallel lines creates: - **Alternate angles** — equal (Z-shape). - **Corresponding angles** — equal (F-shape). - **Co-interior angles** — add to 180° (C-shape). ### Angle addition If D lies inside \(\angle BAC\), then \(m\angle BAC = m\angle BAD + m\angle DAC\). ### Exam tips - Give a **reason** for every step. - Use letters (a, b, c) or Greek letters (α, β, γ, θ) for unknown angles. - Check whether angles are on a line, around a point, or in a shape before calculating.

Angle: A figure formed by two lines meeting at a point; also the size of the opening between them.
Vertex: The point where the two sides of an angle meet.
Acute angle: An angle less than 90°.
Right angle: An angle of exactly 90°.
Obtuse angle: An angle between 90° and 180°.
Straight angle: An angle of exactly 180°.
Reflex angle: An angle between 180° and 360°.
Vertically opposite angles: Angles opposite each other when two lines cross; they are equal.
Alternate angles: Angles on opposite sides of a transversal between parallel lines; they are equal.
Corresponding angles: Angles in matching positions on parallel lines cut by a transversal; they are equal.
Co-interior angles: Angles on the same side of a transversal between parallel lines; they add to 180°.

Thẻ ghi nhớ

What is the vertex of an angle?

The vertex is the point where the two sides (lines or rays) of the angle meet.

How is an angle named using three points, and which point goes in the middle?

Use the angle symbol with three points, e.g. angle BAC, where the vertex is always the middle letter. So angle BAC has vertex A.

What is an acute angle?

An angle smaller than a right angle, so less than 90°.

What is a right angle, and what is the relationship between two lines that form one?

A right angle is 90°, which is 1/4 of a full turn. Two lines that form a right angle are perpendicular (also called normal or orthogonal).

What is an obtuse angle?

An angle larger than a right angle but smaller than a straight angle, so between 90° and 180°.

What is a straight angle?

An angle of 180°, which is 1/2 of a full turn.

What is a reflex angle?

An angle larger than a straight angle but less than a full turn, so between 180° and 360°.

What is a full angle (full turn) in degrees?

A full angle is 360°, which is 1 complete turn.

What is a zero angle?

An angle of 0°, where the two sides coincide (no turn).

What is the angle addition postulate?

If D lies inside angle BAC, then m angle BAC = m angle BAD + m angle DAC. In words, the whole angle equals the sum of the two parts.

What do angles on a straight line add up to?

Angles on a straight line add up to 180°.

What do angles around a point add up to?

Angles around a point add up to 360°.

What are vertically opposite angles, and what is true about them?

When two straight lines cross, the angles opposite each other are vertically opposite. They are equal.

What do the angles in a triangle add up to?

The angles in a triangle add up to 180°.

What do the angles in a quadrilateral add up to?

The angles in a quadrilateral add up to 360°.

What are alternate angles on parallel lines, and what is true about them?

Alternate angles are on opposite sides of the transversal and between the parallel lines, in a Z shape. They are equal.

What are corresponding angles on parallel lines, and what is true about them?

Corresponding angles are in matching positions at each intersection, in an F shape. They are equal.

What are co-interior angles on parallel lines, and what is true about them?

Co-interior angles are on the same side of the transversal between the parallel lines, in a C or U shape. They add up to 180°.

What is a transversal?

A line that crosses two or more other lines. On parallel lines it creates alternate, corresponding and co-interior angle pairs.

Why must you give a reason for each step of an angle calculation?

Each step must be justified by an angle fact, such as angles on a straight line sum to 180° or alternate angles are equal, so the reasoning is clear and correct.

What is the sum of angles on a straight line, and what is the sum of angles around a point?

Angles on a straight line sum to 180°; angles around a point sum to 360°.

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