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Differentiating an Implicit Function: a Circle

  • Thread starter crastinus
  • Start date
  • #1
67
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Homework Statement


Find the expression for the slope on the lower half of the circle y^2 + x^2 = 25.

2. Attempt at a solution.

The text says you get 2x + 2y(dy/dx) = 0.

I got this and then solved for dy/dx to get dy/dx = -2y - 2x.

Then, I substituted for y the x value-expression for the lower region, y = - sqrt(25 - x^2)

and I got dy/dx = -2x - 2(sqrt(25 - x^2)).

Now the text gets the answer in another way:

2x + 2y(dy/dx) = 0;

then, 2x + 2(sqrt(25 - x^2))dy/dx = 0;

then, dy/dx = -2x/2(sqrt(25 - x^2)) = -x/sqrt(25 - x^2).

I see what they did. But what's wrong with the way I did it? Are the two answers equivalent in some way that I don't see, or how is mine wrong?

Thanks.
 

Answers and Replies

  • #2
58
5
The text says you get 2x + 2y(dy/dx) = 0.

I got this and then solved for dy/dx to get dy/dx = -2y - 2x.
Do you notice something wrong here? :)
 
  • #3
67
7
Wow. OK. Yes, I see it. I'm going for a walk!

Thank you very much!
 
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