Implicit Explicit Derivatives
1. **Stating the problem:** Given the implicit equation $$xy + 2x + 3x^2 = -8$$ we need to:
(a) Find $y'$ (the derivative of $y$ with respect to $x$) implicitly.
(b) Solve for $y$ explicitly and differentiate to find $y'$.
2. **Part (a) - Implicit differentiation:**
Differentiate both sides with respect to $x$:
$$\frac{d}{dx}(xy) + \frac{d}{dx}(2x) + \frac{d}{dx}(3x^2) = \frac{d}{dx}(-8)$$
Using product rule on $xy$: $$\frac{d}{dx}(xy) = y + x \frac{dy}{dx} = y + x y'$$
Also, $$\frac{d}{dx}(2x) = 2, \quad \frac{d}{dx}(3x^2) = 6x, \quad \frac{d}{dx}(-8) = 0$$
So the differentiated equation is:
$$y + xy' + 2 + 6x = 0$$
Isolate $y'$:
$$xy' = -y - 2 - 6x$$
$$y' = \frac{-y - 2 - 6x}{x}$$
This is $y'$ implicitly in terms of $x$ and $y$.
3. **Part (b) - Solve explicitly and differentiate:**
Rewrite original equation:
$$xy + 2x + 3x^2 = -8$$
$$xy = -8 - 2x -3x^2$$
Solve for $y$:
$$y = \frac{-8 - 2x - 3x^2}{x} = \frac{-8}{x} - 2 - 3x$$
Differentiate term-by-term:
$$y' = \frac{d}{dx}\left(-8x^{-1} - 2 - 3x\right)$$
$$y' = 8x^{-2} - 0 - 3 = \frac{8}{x^2} - 3$$
4. **Consistency of results:**
From (a): $$y' = \frac{-y - 2 - 6x}{x}$$
Substitute $y = \frac{-8}{x} - 2 - 3x$:
$$y' = \frac{-\left(\frac{-8}{x} - 2 - 3x\right) - 2 - 6x}{x} = \frac{\frac{8}{x} + 2 + 3x - 2 - 6x}{x} = \frac{\frac{8}{x} - 3x}{x} = \frac{8}{x^2} - 3$$
This matches the explicit derivative exactly.
**Final answers:**
(a) $$y' = \frac{-y - 2 - 6x}{x}$$
(b) $$y' = \frac{8}{x^2} - 3$$