NCERT Book Class 7 Maths Lines and Angles

Lines and Angles

5.1 INTRODUCTION

You already know how to identify different lines, line segments and angles in a given shape. Can you identify the different line segments and angles formed in the following figures? (Fig 5.1)

Can you also identify whether the angles made are acute or obtuse or right? Recall that a line segment has two end points. If we extend the two end points in either direction endlessly, we get a line. Thus, we can say that a line has no end points. On the other hand, recall that a ray has one end point (namely its starting point). For example, look at the figures given below: Fig 5.2

Here, Fig 5.2 (i) shows a line segment, Fig 5.2 (ii) shows a line and Fig 5.2 (iii) is that of a ray. A line segment PQ is generally denoted by the symbol PQ, a line AB is denoted by the symbol AB and the ray OP is denoted by OP  Give some examples of line segments and rays from your daily life and discuss them with your friends. Again recall that an angle is formed when lines or line segments meet. In Fig 5.1, observe the corners. These corners are formed when two lines or line segments intersect at a point. For example, look at the figures given below:

In Fig 5.3 (i) line segments AB and BC intersect at B to form angle ABC, and again line segments BC and AC intersect at C to form angle ACB and so on. Whereas, in Fig 5.3 (ii) lines PQ and RS intersect at O to form four angles POS,

SOQ, QOR and ROP. An angle ABC is represented by the symbol

∠ABC. Thus, in Fig 5.3 (i), the three angles formed are ∠ABC, ∠BCA

and ∠BAC, and in Fig 5.3 (ii), the four angles formed are ∠POS, ∠SOQ,

∠QOR and ∠POR. You have already studied how to classify the angles as acute, obtuse or right angle.

Note: While referring to the measure of an angle ABC, we shall write m∠ABC as simply

∠ABC. The context will make it clear, whether we are referring to the angle or its measure.

5.2 RELATED ANGLES

5.2.1 Complementary Angles

When the sum of the measures of two angles is 90°, the angles are called complementary angles.

Please refer to attached file for NCERT Class 7 Maths Lines and Angles