MCQ
Let f(x) = (x - a) (x - b) (x - c), a < b < c. Then f(x) = 0 has two roots. At which interval does these roots belongs?
  • A
    Both the roots in (a, b)
  • B
    At least one root in (a, b) and at least one root in (b, c)
  • C
    Both the roots in (b, c)
  • D
    Neither in (a, b) nor in (b, c)

Answer

  1. At least one root in (a, b) and at least one root in (b, c)

Solution:

f(x) being a polynomial is continuous and differentiable for all real values of x.

We also have f(a) = f(b) = f(c).

If we apply Rolle’s theorem to f(x) in [a, b] and [b, c] we will observe that f(x) = 0

will have at least one root in (a, b) and at least one root in (b, c).

But f(x) is a polynomial of degree two, so that f(x) = 0

can’t have more than two roots. It implies that exactly one root of f(x) = 0

will lie in (a, b) and exactly one root of f(x) = 0 will lie in (b, c).
Let y = f(x) be a polynomial function of degree n. If f(x) = 0 has real roots only,

then f(x) = 0, f(x) = 0, … , fn-1(x) = 0 will have real roots.

It is in fact the general version of above mentioned application,

because if f(x) = 0 have all real roots, then between two consecutive roots of f(x) = 0,

exactly one root of f(x) = 0 will lie.

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