Which term of the A.P. 21, 18, 15, ... is −81?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 35th
The \(n\)th term of an A.P. is \(a_n=a+(n-1)d\). Here the first term \(a=21\) and the common difference \(d=18-21=-3\).
Setting the \(n\)th term equal to \(-81\): \(21+(n-1)(-3)=-81\)
\(-3(n-1)=-102\)
\(n-1=34\), so \(n=35\)