Question

Which term of A.P: 21, 18, 15, . . . . . . . is -81

Answer 1

Answer:

35th term is -81

Step-by-step explanation:

a = 21

d = -3

an = - 81

an = a + (n-1) d

-81 = 21 + (n-1) × -3

-81 -21 = (n-1) × -3

-102/-3 = (n-1)

34 = n-1

34 +1 = n

35 = n

n = 35

Answer 2

EXPLANATION.

• GIVEN

Series of an Ap = 21,18,15,....... is -81

Let first term = 21

common difference = d = b - a = 18 - 21 = -3

Nth term of an Ap = An = -81

Formula of Nth term of an Ap

An = a + ( n - 1 ) d

-81 = 21 + ( n - 1 ) -3

-81 = 21 + ( - 3n + 3 )

-81 = 21 - 3n + 3

-81 = 24 - 3n

-81 - 24 = -3n

- 105 = - 3n

n = 35

Therefore,

Number of terms in ap = 35

35 th term of Ap

T35 = a + 34d

21 + 34( -3)

21 - 102

- 81

Hence verified