JEE Main2017MathematicsSequences and SeriesActual
For any three positive real numbers a , b and c . If 9 25 a 2 + b 2 + 25 c 2 - 3 a c = 15 b 3 a + c . Then
Options
- Ab ,   c and a are in G.P.
- Bb ,   c and a are in A.P.
- Ca ,   b and c are in A.P.
- Da ,   b and c are in G.P.
Correct answer
B. b ,   c and a are in A.P.
Step-by-step solution
225 a 2 + 9 b 2 + 25 c 2 - 75 a c - 45 a b - 15 b c = 0 ⇒     2 15 a 2 + 3 b 2 + 5 c 2 - 15 a 5 c - 15 a 3 b - 3 b 5 c = 0   ⇒     2 15 a 2 + 2 3 b 2 + 2 5 c 2 - 2 15 a 5 c - 2 15 a 3 b - 2 3 b 5 c = 0 ⇒ 15 a - 3 b 2 + 3 b - 5 c 2 + 5 c - 15 a 2 = 0 ⇒ 15 a - 3 b = 3 b - 5 c = 5 c - 15 a = 0 ⇒ 15 a = 3 b = 5 c ⇒     a 1 = b 5 = c 3 = k ⇒       a = k ,   b = 5 k ,   c = 3 k ⇒     a ,   c ,