Question

which terms of the AP: 21,18,15....is -78​

Answer 1

Answer:

-78 is the 34th term in the AP

Step-by-step explanation:

In the given AP first term a= 21

common difference d= -3

The nth term in an AP is An = a+(n-1)d

-78 = 21+(n-1)(-3)

-78-21 = -3n+3

-99 = -3n+3

-99-3 = -3n

-102 = -3n

n = 34

Answer 2

Answer:

34th term

Step-by-step explanation:

Common difference of this AP = 18 - 21

                                                   = - 3

Let -  78 be the nth term of the AP,

Using, a(n) = a + (n - 1)d.

⇒ - 78 = 21 + (n - 1)(-3)

⇒ - 99 = (n - 1)(-3)

⇒ 33 = n - 1

⇒ 34 = n

-78 is the 34th term of the AP