Comprehension

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

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

Consider the following statements:

1. f[g(x)] is a polynomial of degree 3.

2. g[g(x)] is a polynomial of degree 2.

Which of the above statements is/are correct?

This question was previously asked in
NDA (Held On: 17 Nov 2019) Maths Previous Year paper
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  1. 1 only
  2. 2 only
  3. Both 1 and 2
  4. Neither 1 nor 2

Answer (Detailed Solution Below)

Option 4 : Neither 1 nor 2
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Detailed Solution

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Concept:

If p(x) = a0 + a1 x + …… + anxn , where coefficients of x are real and if an ≠ 0 .Then p(x) is a polynomial of degree n.

Calculation:

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

⇒ f[g(x)] = f(5x + 30) = (5x + 30)2 + 2(5x + 30) – 5 = 25x2 + 310x + 955

⇒ f[g(x)] is a polynomial of degree 2.

Hence statement 1 is false.

⇒ g[g(x)] = g(5x + 30) = 5(5x + 30) + 30 = 25x + 180

⇒ g[g(x)] is a polynomial of degree 1.

Hence statement 2 is wrong.
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