Comprehension

Directions: Read the following information and answer the three items that follow:

Let f(x) = x2 + 2x – 5 and g(x) = 5x + 30

What are the roots of the equation g[f(x)] = 0?

This question was previously asked in
NDA (Held On: 17 Nov 2019) Maths Previous Year paper
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  1. 1, -1
  2. -1, -1
  3. 1, 1
  4. 0, 1

Answer (Detailed Solution Below)

Option 2 : -1, -1
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Detailed Solution

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Concept:

If α and β are the roots of the equation f(x) = ax2 + bx + c = 0. Then f(α) = 0 = f(β).

Calculation:

Given: f(x) = x2 + 2x – 5 and g(x) = 5x + 30

⇒ g[f(x)] = g(x2 + 2x – 5) = 5 (x2 + 2x – 5) + 30 = 5x2 + 10x + 5 = 0.

⇒ 5x2 + 10x + 5 = 0

⇒ x2 + 2x + 1 = (x + 1)2 = 0

⇒ x = - 1, -1

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