Problem 19 - Mathematics for IIT JEE Main and Advanced Differential Calculus Algebra Trigonometry

Question

Find the nature of roots of the polynomial \(P(x)=x^{3}+x+1\).

Step-by-Step Solution

Answer

The polynomial \(P(x) = x^3 + x + 1\) has one real root and two complex-conjugate roots, as the discriminant \(\Delta\) is found to be -31, which is negative.

Step 1: Write down the given polynomial

The given polynomial is: \( P(x) = x^3 + x + 1 \)

Step 2: Determine the value of discriminant

The discriminant of a cubic polynomial \( ax^3 + bx^2 + cx + d = 0 \) is given by the formula: \( \Delta = -27a^2d^2 + b^2c^2d^2 - 6abcd + 18abcd - 4ac^3 - 4b^3d \) We substitute the coefficients of given polynomial (\( a = 1, b = 0, c = 1, d = 1 \)) into the discriminant formula, and we get: \( \Delta = -27(1^2)(1^2) + (0^2)(1^2)(1^2) - 6(1)(0)(1)(1) + 18(1)(0)(1)(1) - 4(1)(1^3) - 4(0^3)(1) \)

Step 3: Evaluate the value of the discriminant

Upon calculating, the discriminant simplifies to: \( \Delta = -27(1) + 0 - 0 + 0 - 4(1) - 0 = -31 \)

Step 4: Determine the nature of the roots

Since the discriminant \(\Delta\) is negative, it implies that the polynomial has one real root and two complex-conjugate roots. In conclusion, the polynomial \(P(x) = x^3 + x + 1\) has one real root and two complex-conjugate roots.

Key Concepts

Discriminant of Cubic Polynomial

The discriminant of a cubic polynomial is a critical value that helps us understand the nature of its roots. For any cubic polynomial expressed as \( ax^3 + bx^2 + cx + d = 0 \), the discriminant \( \Delta \) is given by a specific formula:

• \( \Delta = -27a^2d^2 + b^2c^2d^2 - 6abcd + 18abcd - 4ac^3 - 4b^3d \)

The polynomial of interest here is \( P(x) = x^3 + x + 1 \). By identifying the coefficients \( a = 1 \), \( b = 0 \), \( c = 1 \), and \( d = 1 \), we can substitute these values into the discriminant formula. This calculation allows us to see whether the cubic equation has real or complex roots. In this instance, the computation yields \( \Delta = -31 \), a negative number, signaling the nature of its roots.
Understanding how the discriminant guides us in descrying root nature is a stepping stone to mastering polynomial equations in general, especially in fields such as algebra and calculus.

Nature of Polynomial Roots

Once the discriminant is calculated, interpreting its value is crucial in determining the polynomial roots' nature. When it comes to cubic polynomials, the sign of the discriminant is very telling:

• If \( \Delta > 0 \), all three roots are distinct and real.
• If \( \Delta = 0 \), the polynomial has a multiple root, and all roots are real.
• If \( \Delta < 0 \), one root is real, and the other two are complex conjugates.

In our case, the discriminant of \( P(x) = x^3 + x + 1 \) is found to be \( -31 \). This negative discriminant tells us there is just one real root and two others that are complex conjugates.
This concept not only helps in understanding the structure of roots but also guides us in graphing the polynomial or solving equations when exact numeric root values are required.

Complex-Conjugate Roots

When dealing with polynomial equations, especially those with a negative discriminant, complex-conjugate roots become highly relevant. Complex numbers generally appear in pairs called conjugates in these situations, particularly with real coefficients.

• A complex conjugate pair consists of two roots: \( a + bi \) and \( a - bi \).

For the polynomial \( P(x) = x^3 + x + 1 \), with \( \Delta = -31 \), we observe one real root and a pair of these complex conjugate roots. This is because, with a negative discriminant, real coefficients demand that any non-real roots must exist in conjugate pairs.
Complex-conjugate roots have critical importance in many applications, including signal processing and control systems, as they ensure stability and the roots' behavior over real space. Understanding them broadens one's ability to handle both theoretical and practical challenges in mathematics.