- A line segment has two end points.
- A ray has only one end point (its vertex).
- A line has no end points on either side.
| Pairs of Angles | Condition |
| Two complementary angles | Measures add up to 90° |
| Two supplementary angles | Measures add up to 180° |
| Two adjacent angles | Have a common vertex and a common arm but no common interior |
| Linear pair | Adjacent and supplementary |
- A linear pair is a pair of adjacent angles whose non-common sides are opposite rays.
All the types are discussed in detail and are supplemented with examples and short questions.
Under the topic pair of lines, various sub-sections are discussed namely:
- Intersecting lines
- Transversal
- Angles made by a transversal
- When two lines intersect (looking like the letter X) we have two pairs of opposite angles. They are called vertically opposite angles. They are equal in measure.
- A transversal is a line that intersects two or more lines at distinct points.
1. Interior angles
2. Exterior angles
3. Pairs of Corresponding angles
4. Pairs of Alternate interior angles
5. Pairs of Alternate exterior angles
6. Pairs of interior angles on the same side of the transversal
- Transversal of parallel lines
- When a transversal cuts two lines, such that pairs of corresponding angles are equal, then the lines have to be parallel.
- When a transversal cuts two lines, such that pairs of alternate interior angles are equal, the lines have to be parallel.
- When a transversal cuts two lines, such that pairs of interior angles on the same side of the transversal are supplementary, the lines have to be parallel.
The chapter consists of a set of questions to test the basic understanding of the chapter followed by the summary of the discussion for quick recall.
Page No 101:
Question 1:
Find the complement of each of the following angles:
Answer:
The sum of the measures of complementary angles is 90º.
(i) 20°
Complement = 90° − 20°
= 70°
(ii) 63°
Complement = 90° − 63°
= 27°
(iii) 57°
Complement = 90° − 57°
= 33°
Page No 102:
Question 2:
Find the supplement of each of the following angles:
Answer:
The sum of the measures of supplementary anglesis 180º.
(i) 105°
Supplement = 180° − 105°
= 75°
(ii) 87°
Supplement = 180° − 87°
= 93°
(iii) 154°
Supplement = 180° − 154°
= 26°
Page No 102:
Question 3:
Identify which of the following pairs of angles are complementary and which are supplementary.
(i) 65°, 115° (ii) 63°, 27°
(iii) 112°, 68° (iv) 130°, 50°
(v) 45°, 45° (vi) 80°, 10°
Answer:
The sum of the measures of complementary angles is 90º and that of supplementary anglesis 180º.
(i) 65°, 115°
Sum of the measures of these angles = 65º + 115º = 180°
∴ These angles are supplementary angles.
(ii) 63°, 27°
Sum of the measures of these angles = 63º + 27º = 90°
∴ These angles are complementary angles.
(iii) 112°, 68°
Sum of the measures of these angles = 112º + 68º = 180°
∴ These angles are supplementary angles.
(iv) 130°, 50°
Sum of the measures of these angles = 130º + 50º = 180°
∴ These angles are supplementary angles.
(v) 45°, 45°
Sum of the measures of these angles = 45º + 45º = 90°
∴ These angles are complementary angles.
(vi) 80°, 10°
Sum of the measures of these angles = 80º + 10º = 90°
∴ These angles are complementary angles.
Page No 102:
Question 4:
Find the angle which is equal to its complement.
Answer:
Let the angle be x.
Complement of this angle is also x.
The sum of the measures of a complementary angle pair is 90º.
∴ x + x = 90°
2x = 90°
Page No 102:
Question 5:
Find the angle which is equal to its supplement.
Answer:
Let the angle be x.
Supplement of this angle is also x.
The sum of the measures of a supplementary angle pair is 180º.
∴ x + x = 180°
2x = 180°
x = 90°
Page No 102:
Question 6:
In the given figure, ∠1 and ∠2 are supplementary angles. If ∠1 is decreased, what changes should take place in ∠2 so that both the angles still remain supplementary.
Answer:
∠1 and ∠2 are supplementary angles.
If ∠1 is reduced, then ∠2 should be increased by the same measure so that this angle pair remains supplementary.
Page No 102:
Question 7:
Can two angles be supplementary if both of them are:
(i) Acute? (ii) Obtuse? (iii) Right?
Answer:
(i) No. Acute angle is always lesser than 90º. It can be observed that two
angles, even of 89º, cannot add up to 180º. Therefore, two acute angles cannot be in a supplementary angle pair.
(ii) No. Obtuse angle is always greater than 90º. It can be observed that two angles, even of 91º, will always add up to more than 180º. Therefore, two obtuse angles cannot be in a supplementary angle pair.
(iii) Yes. Right angles are of 90º and 90º + 90º = 180°
Therefore, two right angles form a supplementary angle pair together.
Page No 102:
Question 8:
An angle is greater than 45°. Is its complementary angle greater than 45° or equal to 45° or less than 45°?
Answer:
Let A and B are two angles making a complementary angle pair and A is greater than 45º.
A + B = 90º
B = 90º − A
Therefore, B will be lesser than 45º.
Page No 102:
Question 9:
In the adjoining figure:
(i) Is ∠1 adjacent to ∠2?
(ii) Is ∠AOC adjacent to ∠AOE?
(iii) Do ∠COE and ∠EOD form a linear pair?
(iv) Are ∠BOD and ∠DOA supplementary?
(v) Is ∠1 vertically opposite to ∠4?
(vi) What is the vertically opposite angle of ∠5?
Answer:
(i) Yes. Since they have a common vertex O and also a common arm OC. Also, their non-common arms, OA and OE, are on either side of the common arm.
(ii) No. They have a common vertex O and also a common arm OA. However, their non-common arms, OC and OE, are on the same side of the common arm. Therefore, these are not adjacent to each other.
(iii) Yes. Since they have a common vertex O and a common arm OE. Also, their non-common arms, OC and OD, are opposite rays.
(iv) Yes. Since ∠BOD and ∠DOA have a common vertex O and their non-common arms are opposite to each other.
(v) Yes. Since these are formed due to the intersection of two straight lines (AB and CD).
(vi) ∠COB is the vertically opposite angle of ∠5 as these are formed due to the intersection of two straight lines, AB and CD.
Video Solution for lines and angles (Page: 102 , Q.No.: 9)
NCERT Solution for Class 7 math - lines and angles 102 , Question 9
Page No 102:
Question 10:
Indicate which pairs of angles are:
(i) Vertically opposite angles. (ii) Linear pairs.
Answer:
(i) ∠1 and ∠4, ∠5 and ∠2 +∠3 are vertically opposite angles as these are formed due to the intersection of two straight lines.
(ii) ∠1 and ∠5, ∠5 and ∠4 as these have a common vertex and also
have non-common arms opposite to each other.
Page No 103:
Question 11:
In the following figure, is ∠1 adjacent to ∠2? Give reasons.
Answer:
∠1 and ∠2 are not adjacent angles because their vertex is not common.
Page No 103:
Question 12:
Find the value of the angles x, y, and z in each of the following:
|
|
|
|
(i) |
(ii) |
Answer:
(i) Since ∠x and ∠55° are vertically opposite angles,
∠x = 55°
∠x + ∠y = 180° (Linear pair)
55° + ∠y = 180°
∠y = 180º − 55º = 125°
∠y = ∠z (Vertically opposite angles)
∠z = 125°
(ii) ∠z = 40° (Vertically opposite angles)
∠y + ∠z = 180° (Linear pair)
∠y = 180° − 40° = 140°
40° + ∠x + 25° = 180° (Angles on a straight line)
65° + ∠x = 180°
∠x = 180° − 65° = 115°
Video Solution for lines and angles (Page: 103 , Q.No.: 12)
NCERT Solution for Class 7 math - lines and angles 103 , Question 12
Page No 103:
Question 13:
Fill in the blanks:
(i) If two angles are complementary, then the sum of their measures is _______.
(ii) If two angles are supplementary, then the sum of their measures is _______.
(iii) Two angles forming a linear pair are _______.
(iv) If two adjacent angles are supplementary, they form a _______.
(v) If two lines intersect at a point, then the vertically opposite angles are always _______.
(vi) If two lines intersect at a point, and if one pair of vertically opposite angles are acute angles, then the other pair of vertically opposite angles are _______.
Answer:
(i) 90°
(ii) 180°
(iii) Supplementary
(iv) Linear pair
(v) Equal
(vi) Obtuse angles
Page No 103:
Question 14:
In the adjoining figure, name the following pairs of angles.
(i) Obtuse vertically opposite angles
(ii) Adjacent complementary angles
(iii) Equal supplementary angles
(iv) Unequal supplementary angles
(v) Adjacent angles that do not form a linear pair
Answer:
(i) ∠AOD, ∠BOC
(ii) ∠EOA, ∠AOB
(iii) ∠EOB, ∠EOD
(iv) ∠EOA, ∠EOC
(v) ∠AOB and ∠AOE, ∠AOE and ∠EOD, ∠EOD and ∠COD
Page No 110:
Question 1:
State the property that is used in each of the following statements?
(i) If a||b, then ∠1 = ∠5
(ii) If ∠4 = ∠6, then a||b
(iii) If ∠4 + ∠5 = 180°, then a||b
Answer:
(i) Corresponding angles property
(ii) Alternate interior angles property
(iii) Interior angles on the same side of transversal are supplementary.
Page No 110:
Question 2:
In the adjoining figure, identify
(i) The pairs of corresponding angles
(ii) The pairs of alternate interior angles
(iii) The pairs of interior angles on the same side of the transversal
(iv) The vertically opposite angles
Answer:
(i) ∠1 and ∠5, ∠2 and ∠6, ∠3 and ∠7, ∠4 and ∠8
(ii) ∠2 and ∠8, ∠3 and ∠5
(iii) ∠2 and ∠5, ∠3 and ∠8
(iv) ∠1 and ∠3, ∠2 and ∠4, ∠5 and ∠7, ∠6 and ∠8
Video Solution for lines and angles (Page: 110 , Q.No.: 2)
NCERT Solution for Class 7 math - lines and angles 110 , Question 2
Page No 110:
Question 3:
In the adjoining figure, p || q. Find the unknown angles.
Answer:
∠d = 125° (Corresponding angles)
∠e = 180° − 125° = 55° (Linear pair)
∠f = ∠e = 55° (Vertically opposite angles)
∠c = ∠f = 55° (Corresponding angles)
∠a = ∠e = 55° (Corresponding angles)
∠b = ∠d = 125° (Vertically opposite angles)
Video Solution for lines and angles (Page: 110 , Q.No.: 3)
NCERT Solution for Class 7 math - lines and angles 110 , Question 3
Page No 111:
Question 4:
Find the value of x in each of the following figures if l || m.
Answer:
(i)
∠y = 110° (Corresponding angles)
∠x + ∠y = 180° (Linear pair)
∠y = 180° − 110°
= 70°
(ii)
∠x = 100° (Corresponding angles)
Page No 111:
Question 5:
In the given figure, the arms of two angles are parallel.
If ∠ABC = 70°, then find
(i) ∠DGC
(ii) ∠DEF
Answer:
(i) Consider that AB|| DG and a transversal line BC is intersecting them.
∠DGC = ∠ABC (Corresponding angles)
∠DGC = 70°
(ii) Consider that BC|| EF and a transversal line DE is intersecting them.
∠DEF = ∠DGC (Corresponding angles)
∠DEF = 70°
Video Solution for lines and angles (Page: 111 , Q.No.: 5)
NCERT Solution for Class 7 math - lines and angles 111 , Question 5
Page No 111:
Question 6:
In the given figures below, decide whether l is parallel to m.
Answer:
(i)
Consider two lines, l and m, and a transversal line n which is intersecting them.
Sum of the interior angles on the same side of transversal = 126º + 44º = 170°
As the sum of interior angles on the same side of transversal is not 180º, therefore, l is not parallel to m.
(ii)
x + 75° = 180° (Linear pair on line l)
x = 180° − 75° = 105°
For l and m to be parallel to each other, corresponding angles (∠ABC and ∠x)should be equal. However, here their measures are 75º and 105º respectively. Hence, these lines are not parallel to each other.
(iii)
∠x + 123° = 180° (Linear pair)
∠x = 180° − 123º = 57°
For l and m to be parallel to each other, corresponding angles (∠ABC and ∠x)should be equal. Here, their measures are 57º and 57º respectively. Hence, these lines are parallel to each other.
98 + ∠x = 180° (Linear pair)
∠x = 82°
For l and m to be parallel to each other, corresponding angles (∠ABC and ∠x)should be equal. However, here their measures are 72º and 82º respectively. Hence, these lines are not parallel to each other.
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