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## Homework Statement

Find equations of both the tangent lines to the ellipse

x

^{2}+ 4y

^{2}= 36

that pass through the point (12, 3).

## Homework Equations

The equation of an ellipse is x

^{2}/a

^{2}+ y

^{2}/b

^{2}= 1.

I converted the given equation to

x

^{2}/36 + y

^{2}/9 = 1 by dividing each value by 36.

## The Attempt at a Solution

My online homework website is telling me I'm half right (I found the equation of ONE of the tangent lines to the ellipse passing through (12,3)). Here's what I have:

I took the derivative of the equation of the ellipse and got y' = -x/4y.

I set this value equal to the slope of a line passing through the point (12,3), (y-3)/(x-12). I then solved for x

^{2}and y

^{2}to find

x

^{2}=-4y

^{2}+ 12y + 12x

y

^{2}=-x

^{2}/4 + 3x + 3y

I then substituted these equations, one at a time, for the values of x

^{2}and y

^{2}in the original equation of the ellipse and solved for the unknown value. I think I took an unnecessary step doing that, but I need the practice! I found that

x=0

y=±3

I then determined the slopes of the lines passing through (0,3)-->(12,3) and (0,-3)-->(12,3). Clearly, the line running from (0,3)-->(12,3) has a slope of 0, and the equation of the line is y=3. This is the part that I got correct.

The slope of the line running through (0,-3)-->(12,3) I found to be 1/2. However, my homework website is telling me the equation y=1/2x-3 is incorrect.

Any assistance you can give me would be very appreciated! Honestly, we haven't gone over ANY problems like this in class, so the fact I'm even half right has been enormously satisfying. But I'd like to know what I'm doing wrong... I have a feeling I probably did something stupid, like dropped a negative sign somewhere. There are only 6 questions on this section's homework, so as it is I have an 83.3% and can't make an A unless I get this last piece of the last question correct. So frustrating!

Thanks in advance!