Question 12

Mathematics Differentiation Hard

The equation \( (x - a)^3 + (x - b)^3 + (x - c)^3 \) = 0 has

(A) All three real roots
(B) One real and two imaginary root
(C) Three real roots, namely x = a, y = b, z = c
(D) None of these
View Dynamic Solution & Explanation
Correct Solution: Option B

Step-by-step Solution:

Concept:

Nature of roots of a cubic polynomial:

  • If the maximum and minimum are of opposite signs, the cubic has three real roots.
  • If one of them is zero, two of the three roots are equal.
  • If both of them are zero, all three roots are equal.
  • If the maximum and minimum are of the same sign, the cubic has one real and two imaginary roots.

 

Calculations:

Given equation is (x - a)3 + (x - b)3 + (x - c)3 = 0

Consider, f(x) = (x - a)3 + (x - b)3 + (x - c)3

Differentiating both sides, we get

f'(x) = 3(x - a)2 + 3(x - b)2 + 3(x - c)2

f'(x) > 0

Hence, f is an increasing function.

To find the maximum and minimum of the function, put x = ∞ in f(x):

Now, f(∞) = ∞

f(-∞) = ∞

Here, the maximum and minimum of the function are of the same sign.

Hence, the equation (x - a)3 + (x - b)3 + (x - c)3 = 0 has one real and two imaginary roots.