Question

If 2X + 1 = 3X² then find value of 3X² + 5X + 7. Note X > 0

1. 18
2. 15
3. 16
4. 17

Answer 1

Correct Answer - Option 2 : 15

Calculation:

2X + 1 = 3X²

⇒ D = 4 + 12 (∵ D = b² – 4ac)

⇒ D = 16

⇒ √ D = 4

α = (-b + √ D)/ 2a

⇒ α = (2 + 4)/ 6

⇒ α = 1

β = (-b - √ D)/ 2a

⇒ β = - 2/ 6

⇒ β = -1/ 3

Here, X = 1

Now, 3X² + 5X + 7

⇒ 3 + 5 + 7   (For X = 1)

⇒ 15

∴ Required answer is 15

 

For equation ax² + bx + c 

Roots = α and β 

α = (-b + √ D)/ 2a

β = (-b - √ D)/ 2a