Question

15th term of A.P 21,16,11,6​

Answer 1

a=21

a2=16

d=a2-a

d=16-21

d=-5

a15=a+(15-1)d

a15=21+(14)-5

a15=21-70

a15=-49

Answer 2

15th term of A.P 21,16,11,6​, ... is -49

Given:

AP 21,16,11,6​, ...

To Find:

15th term

Solution:

Arithmetic sequence

Sequence of terms in which difference between one term and the next is a constant.

This is also called Arithmetic Progression AP

Arithmetic sequence can be represented in the form :

a, a + d  , a + 2d , …………………………, a + (n-1)d

a = First term

d = common difference = aₙ-aₙ₋₁

nth term   aₙ =  a + (n-1)d  

Sₙ = (n/2)(2a + (n - 1)d)

Step 1:

Find the Common difference

16 - 21 = 11 - 16 = 6 - 11 = - 5

Step 2:

Use nth term formula   aₙ =  a + (n-1)d   and substitute a = 21 , n = 15 , d = -5

a₁₅ = 21 + (15 - 1)(-5)

=> a₁₅ = 21 - 70

=> a₁₅ = -49

15th term of A.P 21,16,11,6​, ...  is -49