Examples

Chapter 5 Class 10 Arithmetic Progressions
Serial order wise

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### Transcript

Example 4 Which term of the AP : 21, 18, 15, . . . is –81? Also, is any term 0? Give reason for your answer. Given AP 21, 18, 15, …………. We need to find which term is – 81. Thus, an = – 81 & a = 21 & d = 18 – 21 = –3 We find to find n Putting these in formula an = a + (n – 1) d – 81 = 21 + (n – 1) (–3) – 81 = 21 + n (–3) – 1(–3) – 81 = 21 – 3n + 3 – 81 = 24 – 3n 3n = 24 + 81 3n = 105 n = 105/3 n = 35 Therefore, the 35th term of given AP is –81 Now we have to find which term is 0. an = a + (n – 1)d Here an = 0 , a = 21 & d = –3 We find to find n Putting these in formula an = a + (n – 1) d 0 = 21 + (n – 1) (–3) 0 = 21 + n ( – 3) – 1(–3) 0 = 21 – 3n + 3 0 = 24 – 3n 3n = 24 n = 24/3 n = 8 Therefore, the 8th term of given AP is 0,

#### Davneet Singh

Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 13 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.