Sign in
Please select an account to continue using cracku.in
↓ →
Join Our JEE Preparation Group
Prep with like-minded aspirants; Get access to free daily tests and study material.
If $$2 + 3i$$ is one of the roots of the equation $$2x^3 - 9x^2 + kx - 13 = 0$$, $$k \in R$$, then the real root of this equation (where $$i^2 = -1$$):
Given that $$2 + 3i$$ is a root of the equation $$2x^3 - 9x^2 + kx - 13 = 0$$ and $$k$$ is real, we know that complex roots occur in conjugate pairs for polynomials with real coefficients. Therefore, the conjugate $$2 - 3i$$ must also be a root.
Let the roots be $$\alpha = 2 + 3i$$, $$\beta = 2 - 3i$$, and $$\gamma$$ (the real root). The cubic equation can be expressed as $$2(x - \alpha)(x - \beta)(x - \gamma) = 0$$. First, compute the quadratic factor from the complex roots:
$$(x - \alpha)(x - \beta) = (x - 2 - 3i)(x - 2 + 3i) = [(x - 2) - 3i][(x - 2) + 3i] = (x - 2)^2 - (3i)^2$$
Since $$i^2 = -1$$, we have:
$$(x - 2)^2 - 9i^2 = (x^2 - 4x + 4) - 9(-1) = x^2 - 4x + 4 + 9 = x^2 - 4x + 13$$
So the equation becomes:
$$2(x^2 - 4x + 13)(x - \gamma) = 2x^3 - 9x^2 + kx - 13$$
Expand the left side:
$$2(x^2 - 4x + 13)(x - \gamma) = 2\left[ x^2(x - \gamma) - 4x(x - \gamma) + 13(x - \gamma) \right] = 2\left[ x^3 - \gamma x^2 - 4x^2 + 4\gamma x + 13x - 13\gamma \right]$$
Combine like terms:
$$= 2\left[ x^3 - (4 + \gamma)x^2 + (4\gamma + 13)x - 13\gamma \right] = 2x^3 - 2(4 + \gamma)x^2 + 2(4\gamma + 13)x - 2(13\gamma)$$
Simplify:
$$= 2x^3 - (8 + 2\gamma)x^2 + (8\gamma + 26)x - 26\gamma$$
Equate this to the given cubic $$2x^3 - 9x^2 + kx - 13$$ and compare coefficients:
For $$x^2$$: $$- (8 + 2\gamma) = -9$$
For $$x$$: $$8\gamma + 26 = k$$
For constant term: $$-26\gamma = -13$$
Solve the constant term equation for $$\gamma$$:
$$-26\gamma = -13$$
Divide both sides by $$-13$$:
$$\frac{-26\gamma}{-13} = \frac{-13}{-13} \implies 2\gamma = 1 \implies \gamma = \frac{1}{2}$$
Verify with the $$x^2$$ coefficient equation:
$$- (8 + 2\gamma) = -9$$
Substitute $$\gamma = \frac{1}{2}$$:
$$- \left(8 + 2 \times \frac{1}{2}\right) = - (8 + 1) = -9$$
This holds true. The $$x$$ coefficient gives $$k$$, but it is not needed for finding the real root.
Thus, the real root $$\gamma$$ is $$\frac{1}{2}$$. Checking the options:
A. Exists and is equal to $$\frac{1}{2}$$
B. Does not exist
C. Exists and is equal to 1
D. Exists and is equal to $$-\frac{1}{2}$$
Hence, the correct answer is Option A.
Create a FREE account and get:
Educational materials for JEE preparation