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MTH 102 MTH 102 Past Questions Objective Question

Question

Find $\frac{dy}{dx}$ if $3x^2 y + xy^2 = 5$
Options
A $\frac{y^2+6xy}{3x^2+2xy}$
B $-\frac{y^2+6xy}{3x^2+2xy}$
Correct Answer
C $-\frac{y^2+6xy}{3x^2-2xy}$
D $\frac{y^2-6xy}{3x^2-2xy}$
Correct Answer

Option B is the correct answer.

Detailed Explanation

We need to differentiate the equation implicitly with respect to $x$.$\frac{d}{dx}(3x^2 y) + \frac{d}{dx}(xy^2) = \frac{d}{dx}(5)$Apply the product rule for differentiation to the terms involving $y$:For $3x^2 y$: $(6x)y + 3x^2 \frac{dy}{dx}$For $xy^2$: $(1)y^2 + x(2y \frac{dy}{dx})$For $5$: $0$Substitute these back into the equation:$6xy + 3x^2 \frac{dy}{dx} + y^2 + 2xy \frac{dy}{dx} = 0$Group terms containing $\frac{dy}{dx}$:$(3x^2 + 2xy) \frac{dy}{dx} = -6xy - y^2$Isolate $\frac{dy}{dx}$:$\frac{dy}{dx} = \frac{-6xy - y^2}{3x^2 + 2xy}$Factor out $-1$ from the numerator:$\frac{dy}{dx} = -\frac{y^2 + 6xy}{3x^2 + 2xy}$This matches option B.

Hint

Use implicit differentiation and the product rule. Remember to treat $y$ as a function of $x$, so its derivative is $\frac{dy}{dx}$.

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