Question

20th term from the last term of the AP 21,18,15...., -72 is​

Answer 1

Step-by-step explanation:

a = 21

d = 18-21 = -3

a

$a_{n} = - 72 \\ a_{n} = a + (n - 1) \times d \\ - 72 = 21 + (n - 1) \times ( - 3) \\ - 72 - 21 = - 3n + 3 \\ - 96 = - 3n \\ - 96 \div - 3 = n \\ 32 = n \\ \\ \\ 20th \: term \: from \: last \: = 32 - 19 = 23 term \\ a _{23} = a + (n - 1) \times d \\ \: \: \: \: \: \: \: \: = 21 + (23 - 1) \times ( - 3) \\ = 21 - 66 \\ a _{23} = - 45$