Calculating Lamp Location Above x-Axis: Elliptical Shadow Problem

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SUMMARY

The discussion centers on calculating the height of a lamp located three units to the right of the y-axis, which casts a shadow defined by the elliptical equation x² + 4y² = 5. The point (-5, 0) lies on the edge of the shadow, leading to the conclusion that the tangent line at this point is undefined since the ellipse does not include (-5, 0). The solution involves finding the point of tangency (x₀, y₀) that satisfies both the slope condition derived from the derivative y' = -x/(4y) and the ellipse equation.

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Homework Statement


A lamp is located three units to the right of the y-axis and a shadow is created by the elliptical region x^2 + 4y^2 = 5. If the point (-5,0) is on the edge of the shadow, how far above the x-axis is the lamp located?

2. The attempt at a solution
Ive calculated the derivative with respect to X to be -x/4y = y'. But when i try to find the slope, I am stuck with an undefined when i try to use the point (-5,0)

Any help would be appriciated. Thanks.
 
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Well, the ellipse itself does not include the point (-5, 0) so there is no tangent to the ellipse there. What you want is a line through (-5, 0) that is tangent to the ellipse at some other point.

Let [itex](x_0,y_0)[/itex] be the point of tangency. The line through (-5, 0) and [itex](x_0, y_0)[/itex] has slope [itex](y_0- 0)/(x_0-(-5))= y_0/(x_0+ 5)[/itex].

Yes, the derivative is given by [itex]y'= -x/(4y)[/itex] and at [itex](x_0, y_0)[/itex] that is [itex]-x_0/(4y_0)[/itex].

So you want to find [itex](x_0, y_0)[/itex] satisfying both [itex]y_0/(x_0+ 5)= -x_0/(4y_0)[/itex] and the equation of the ellipse, [itex]x_0^2+ 4y_0^2= 5[/itex]
 

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