JEE Main 2024 Mathematics question
Let denote the sum of the first n terms of an arithmetic progression. If = 390 and the ratio of the tenth the fifth terms is 15 : 7, then is equal to:
Answer: (C)
Solution
(a + 9d)/(a + 4d) = 15/7 ⇒ 3d = 8a; S₁₀ = 390 gives a = 3, d = 8; S₁₅ − S₅ = 885 − 95 = 790. (The source omits "term" after "tenth".)