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The slope of the tangent to curve is (xy...

The slope of the tangent to curve is `(xy^2+y)/x` and it intersects the line `x+2y=4` at `x=-2. If (3,y)` lies on the curve then y is

A

`-18/19`

B

119

C

`-18/29`

D

None of these

Text Solution

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The correct Answer is:
To solve the problem, we need to find the value of \( y \) such that the point \( (3, y) \) lies on the curve defined by the slope of the tangent, which is given as \( \frac{xy^2 + y}{x} \). We also know that this curve intersects the line \( x + 2y = 4 \) at \( x = -2 \). ### Step-by-Step Solution: 1. **Understanding the Slope of the Tangent:** The slope of the tangent to the curve is given by: \[ \frac{dy}{dx} = \frac{xy^2 + y}{x} \] 2. **Rearranging the Slope Equation:** We can rearrange this equation: \[ x \frac{dy}{dx} = xy^2 + y \] This can be rewritten as: \[ x \frac{dy}{dx} - y = xy^2 \] 3. **Separating Variables:** We can separate variables: \[ x \frac{dy}{dx} = xy^2 + y \implies \frac{dy}{y^2 + \frac{y}{x}} = \frac{dx}{x} \] 4. **Integrating Both Sides:** Integrating both sides, we have: \[ \int \frac{dy}{y^2 + \frac{y}{x}} = \int \frac{dx}{x} \] This integration will yield: \[ -\frac{x}{y} = \frac{x^2}{2} + C \] 5. **Finding the Intersection with the Line:** The line \( x + 2y = 4 \) can be rearranged to find \( y \): \[ 2y = 4 - x \implies y = \frac{4 - x}{2} \] At \( x = -2 \): \[ y = \frac{4 - (-2)}{2} = \frac{6}{2} = 3 \] Thus, the curve intersects the line at the point \( (-2, 3) \). 6. **Substituting the Intersection Point:** Now we substitute \( (-2, 3) \) into the curve equation: \[ -\frac{-2}{3} = \frac{(-2)^2}{2} + C \] Simplifying gives: \[ \frac{2}{3} = 2 + C \implies C = \frac{2}{3} - 2 = \frac{2}{3} - \frac{6}{3} = -\frac{4}{3} \] 7. **Final Curve Equation:** The equation of the curve becomes: \[ -\frac{x}{y} = \frac{x^2}{2} - \frac{4}{3} \] 8. **Finding \( y \) when \( x = 3 \):** Substitute \( x = 3 \) into the curve equation: \[ -\frac{3}{y} = \frac{3^2}{2} - \frac{4}{3} \] This simplifies to: \[ -\frac{3}{y} = \frac{9}{2} - \frac{4}{3} \] Finding a common denominator (6): \[ -\frac{3}{y} = \frac{27}{6} - \frac{8}{6} = \frac{19}{6} \] Cross-multiplying gives: \[ -3 \cdot 6 = 19y \implies -18 = 19y \implies y = -\frac{18}{19} \] ### Conclusion: The value of \( y \) such that the point \( (3, y) \) lies on the curve is: \[ \boxed{-\frac{18}{19}} \]
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