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The graph of the equation 2x2 + xy + y2 = 4 is the tilted ellipse pictured below; i.e. the points (x,y) in the plane that satisfy the equation yield the pictured ellipse. The point P=(1,-2) on the ellipse and the tangent through P are pictured below. A line that is tangent to an ellipse does not intersect the ellipse in any other point. We can use this fact to determine the slope of the tangent line.

Verified Answer :

The slope of the tangent at point \(P = \left( {1, - 2} \right)\) is \(\boxed{\frac{2}{3}}.\)


Further explanation:


Given:

The equation of the ellipse is \(2{x^2} + xy + {y^2} = 4.\)

The point \(P = \left( {1, - 2} \right)\) is on the ellipse and the tangent.


Explanation:

The given equation of the ellipse is \(2{x^2} + xy + y = 4.\)

Now differentiate the equation of ellipse with respect to \(x\).

\(\begin{aligned}\frac{d}{{dx}}\left( {2{x^2} + xy + {y^2}} \right) &= \frac{d}{{dx}}\left( 4 \right)\\\frac{d}{{dx}}\left( {2{x^2}} \right) + \frac{d}{{dx}}\left( {xy} \right) + \frac{d}{{dx}}\left( {{y^2}} \right)&= \frac{d}{{dx}}\left( 4 \right)\\4x + x \times \frac{{dy}}{{dx}} + y\frac{d}{{dx}}\left( x \right) + 2y\frac{{dy}}{{dx}}&= 0\\4x + y + \left( {x + 2y} \right)\frac{{dy}}{{dx}}&= 0\\4x + y + \left( {x + 2y} \right)\frac{{dy}}{{dx}} &= 0 \\\end{aligned}\)


Further solve the above equation.

\(\begin{aligned}\left( {x + 2y}\right)\frac{{dy}}{{dx}}&=- 4x - y\\\frac{{dy}}{{dx}}&=-\frac{{4x + y}}{{\left( {x + 2y} \right)}}\\\end{aligned}\)


Substitute \(1\) for \(x\) and \(-2\) for y in equation \(\dfrac{{dy}}{{dx}}= - \dfrac{{4x + y}}{{x + 2y}}.\)

\(\begin{aligned}\frac{{dy}}{{dx}} &= - \frac{{4\left( 1 \right) + \left( { - 2} \right)}}{{1 + 2\left( { - 2} \right)}} \\&= - \frac{{4 - 2}}{{1 - 4}}\\&= - \frac{2}{{ - 3}}\\&= \frac{2}{3} \\\end{aligned}\)


The slope of the tangent at point \(P = \left( {1, - 2} \right)\) is \(\boxed{\frac{2}{3}}.\)


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Answer details:

Grade: High School

Subject: Mathematics

Chapter: Application of derivatives


Keywords: Derivative, graph of the equation, 2x2+xy+y2=4, ellipse, tangent, equation yield, pictured ellipse, line, intersect, slope, normal, slope of tangent line.

The slope of the tangent line through the point P(1, -2) is;

Slope = 2/3

We are given the equation of an ellipse as;

2x² + xy + y² = 4

   

     Now, in mathematics we know that the slope of a tangent is also the same as the first derivative. Thus, let us differentiate the equation of the ellipse with respect to x. Thus;

4x(dx) + x(dy) + y(dx) + 2y(dy) = 0

Collecting like terms;

(4x + y)dx + (x + 2y)dy = 0

⇒ (4x + y)dx = - (x + 2y)dy

divide both sides by dx to get;

(4x + y) = - (x + 2y)(dy/dx)

dy/dx = (4x + y)/-(x + 2y)

     At point P(1, -2), the slope is;

dy/dx =  (4(1) + -2)/-(1 + 2(-2))

dy/dx = 2/3

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