If the first term of an A.P. is 3 and the sum of its first four terms is equal to one-fifth of the sum of the next four terms, then the sum of the first 20 terms is equal to
- -120
- -1200
- -1080
- -1020
Answer
(3) -1080
Solution
S4= 15 (S8-S4 ) S8=6S4
a=3: 24+28d=6(12+6d) d=-6
S20= 202 [6+19(-6) ]=-1080
Answer: (3)