Question:easy

For which values of \(k\) does the function \(f(z)=\left(x^3-3xy^2\right)+i\left(kx^2y-y^3\right)\) become analytic?

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A complex function \(f(z)=u+iv\) is analytic when it satisfies the Cauchy-Riemann equations: \(u_x = v_y\) and \(u_y = -v_x\). We match the partial derivatives of the real and imaginary parts to find k.
Updated On: Jun 16, 2026
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The Correct Option is D

Solution and Explanation

Step 1: Name the real and imaginary parts.
Here \(u=x^3-3xy^2\) and \(v=kx^2y-y^3\). The function is analytic when the Cauchy-Riemann equations hold.

Step 2: Differentiate u.
\(u_x=3x^2-3y^2\) and \(u_y=-6xy\).

Step 3: Differentiate v.
\(v_x=2kxy\) and \(v_y=kx^2-3y^2\).

Step 4: Apply the first CR equation \(u_x=v_y\).
\(3x^2-3y^2=kx^2-3y^2\), so \(3x^2=kx^2\), giving \(k=3\).

Step 5: Check the second CR equation \(u_y=-v_x\).
\(-6xy=-2kxy\Rightarrow 6=2k\Rightarrow k=3\). Both equations agree.

Step 6: State the answer.
The function is analytic only when \(k=3\), which is option 4.
\[ \boxed{k=3} \]
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