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Question
Which term of the AP 21, 18, 15, …… is -81?
Solution
The given AP is 21,18,15,.......
First term, a = 21
Common difference, d= 18 -21 =-3
Suppose nth term of the given AP is - 81. then,
an = -81
⇒ 21 +(n-1) × (-3) =-81 [an = a + (n-1)d]
⇒ -3 (n-1) = -81 -21=-102
⇒ `n-1 = 102/3 = 34`
⇒ n= 34+1=35
Hence, the 35th term of the given AP is -81.
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