The second and the last term of an A.P. are 18 and 720 respectively and their common difference is 6. What is their sum?
Correct Answer: Option(b)
Solution: Given that, a2=18; last term, l=720,d=6 d=a2−a1 6=18−a1 a1=12 We know that, last term, l=a+(n−1)d ⇒720=12+(n−1)6 ⇒720=12+6n−6 ⇒720−6=6n ⇒n=6714 ⇒n=119 We know Sn=2n [First term + Last term] =2119[12+720] =59.5 Therefore, Sn=43,554