Question

Which term of the AP: 21, 18, 15, . . . is – 81? Also, is any term 0? Give reason for your answer.
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Answer 1

Answer:

-81 is the 35th term of the given A.P

Explanation:

given AP, 21,18,15,....

=> a, first term = 21

&  d, common difference = t₂-t₁ = 18-21 = -3

given tₙ = -81

we know, tₙ = a + (n-1)d   , where tₙ is the n'th term

          => -81 = 21 + (n-1)(-3)

          => -81 = 21 + -3n + 3

          => 3n = 21 +81 + 3

          => 3n = 105

          => n = 105/3

          => n = 35

=> -81 is the 35th term of the given A.P