If $x^2 + y^2 = 25$, then $\dfrac{dy}{dx}$ equals:
A$x/y$
B$-x/y$
C$-y/x$
D$y/x$
Answer & Solution
Correct answer: B. $-x/y$
Implicit differentiation: $2x + 2y\,dy/dx = 0$ ⇒ $dy/dx = -x/y$. (Circle: slope of tangent.)
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