Question Detail
Question
The 5th term of an AP is 20 and the 12th term is 48. Find the sum of the first 15 terms.
Correct Answer
Option A is the correct answer.
Detailed Explanation
t_5=a+4d=20, t_12=a+11d=48 → 7d=28 → d=4 → a=20−16=4 → S_15=15/2[2×4+14×4]=7.5[8+56]=7.5×64=480.
Hint
Find a and d first, then use S_n.