Originally Posted by

**NBrunk** We covered implicit differentiation recently, and I'm relatively clear on it, but I'd like to clarify a few things.

I'll first explain the example and then explain my questions.

Example I

(a) If x^2 + y^2 = 25, find dy/dx

(b) Find an equation of the tangent line to the circle x^2 +y^2 = 25 at the point (3, 4).

(a) Questions: I'm not quite clear on why the derivative of y^2 = 2y * y' rather than simply 2y. I understand that it's because y is a function of x, but don't understand why multiplying by y' is the solution.