Implicit Derivative And Integration
1. **Problem 1: Find \( \frac{dy}{dx} \) given the implicit equation \( x^2 + xy + y^3 = 0 \).**
2. We use implicit differentiation. Differentiate both sides with respect to \( x \):
$$\frac{d}{dx}(x^2) + \frac{d}{dx}(xy) + \frac{d}{dx}(y^3) = \frac{d}{dx}(0)$$
3. Apply the rules:
- \( \frac{d}{dx}(x^2) = 2x \)
- For \( xy \), use product rule: \( \frac{d}{dx}(xy) = x \frac{dy}{dx} + y \)
- For \( y^3 \), use chain rule: \( 3y^2 \frac{dy}{dx} \)
4. Substitute:
$$2x + x \frac{dy}{dx} + y + 3y^2 \frac{dy}{dx} = 0$$
5. Group terms with \( \frac{dy}{dx} \):
$$x \frac{dy}{dx} + 3y^2 \frac{dy}{dx} = -2x - y$$
6. Factor out \( \frac{dy}{dx} \):
$$\frac{dy}{dx}(x + 3y^2) = -2x - y$$
7. Solve for \( \frac{dy}{dx} \):
$$\frac{dy}{dx} = -\frac{2x + y}{x + 3y^2}$$
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8. **Problem 2: Given \( f'(x) = e^x + \cos x \) and \( f(0) = 0 \), find \( f(x) \).**
9. To find \( f(x) \), integrate \( f'(x) \):
$$f(x) = \int (e^x + \cos x) dx = \int e^x dx + \int \cos x dx$$
10. Integrate each term:
$$\int e^x dx = e^x + C_1$$
$$\int \cos x dx = \sin x + C_2$$
11. Combine constants into one \( C \):
$$f(x) = e^x + \sin x + C$$
12. Use initial condition \( f(0) = 0 \):
$$0 = e^0 + \sin 0 + C = 1 + 0 + C \implies C = -1$$
13. Final function:
$$f(x) = e^x + \sin x - 1$$