Derivatives Calculus
1. Problem: Find the derivative of $$y = (2x^2 - 4x^3)^4$$.
Step 1: Use the chain rule. Let $$u = 2x^2 - 4x^3$$, so $$y = u^4$$.
Step 2: Compute $$\frac{dy}{du} = 4u^3$$.
Step 3: Compute $$\frac{du}{dx} = 4x - 12x^2$$.
Step 4: Apply the chain rule: $$\frac{dy}{dx} = \frac{dy}{du} \cdot \frac{du}{dx} = 4(2x^2 - 4x^3)^3 (4x - 12x^2)$$.
2. Problem: Find all existing higher derivatives of $$y = x^5 - 2x^4 + x^3 - 3x^2 - 4x + 7$$.
Step 1: First derivative:
$$y' = 5x^4 - 8x^3 + 3x^2 - 6x - 4$$
Step 2: Second derivative:
$$y'' = 20x^3 - 24x^2 + 6x - 6$$
Step 3: Third derivative:
$$y''' = 60x^2 - 48x + 6$$
Step 4: Fourth derivative:
$$y^{(4)} = 120x - 48$$
Step 5: Fifth derivative:
$$y^{(5)} = 120$$
Step 6: Sixth derivative and higher derivatives:
$$y^{(6)}= 0$$ and all higher derivatives are zero.
3. Problem: Find the derivative of $$y = \sqrt{x^2 + 1}$$.
Step 1: Rewrite: $$y = (x^2 + 1)^{1/2}$$.
Step 2: Use the chain rule:
$$\frac{dy}{dx} = \frac{1}{2}(x^2 + 1)^{-1/2} \cdot 2x = \frac{x}{\sqrt{x^2 + 1}}$$.
4. Problem: Given $$y = \frac{u^2}{u^2 + 1}$$ and $$u = \sqrt{2x + 1}$$.
(i) Find $$\frac{dy}{dx}$$ by the chain rule.
Step 1: Rewrite $$y$$ as $$y = \frac{u^2}{u^2 + 1}$$.
Step 2: Compute $$\frac{dy}{du}$$ using quotient rule:
$$\frac{dy}{du} = \frac{(2u)(u^2 + 1) - u^2(2u)}{(u^2 + 1)^2} = \frac{2u^3 + 2u - 2u^3}{(u^2 + 1)^2} = \frac{2u}{(u^2 + 1)^2}$$.
Step 3: Compute $$\frac{du}{dx}$$:
$$u = (2x + 1)^{1/2}, \quad \frac{du}{dx} = \frac{1}{2}(2x+1)^{-1/2} \cdot 2 = \frac{1}{\sqrt{2x + 1}}$$.
Step 4: Chain rule:
$$\frac{dy}{dx} = \frac{dy}{du} \cdot \frac{du}{dx} = \frac{2u}{(u^2 + 1)^2} \cdot \frac{1}{\sqrt{2x + 1}}$$.
Step 5: Substitute back $$u = \sqrt{2x + 1}$$:
$$\frac{dy}{dx} = \frac{2 \sqrt{2x + 1}}{((2x + 1) + 1)^2} \cdot \frac{1}{\sqrt{2x + 1}} = \frac{2}{(2x + 2)^2} = \frac{2}{4(x+1)^2} = \frac{1}{2(x+1)^2}$$.
(ii) Construct the composite function and then find $$\frac{dy}{dx}$$.
Step 1: Substitute $$u = \sqrt{2x + 1}$$ into $$y$$:
$$y = \frac{(\sqrt{2x + 1})^2}{(\sqrt{2x + 1})^2 + 1} = \frac{2x + 1}{2x + 1 + 1} = \frac{2x + 1}{2x + 2}$$.
Step 2: Simplify denominator:
$$y = \frac{2x + 1}{2(x + 1)}$$.
Step 3: Find derivative using quotient rule:
$$\frac{dy}{dx} = \frac{2 \cdot 2(x+1) - (2x + 1) \cdot 2}{(2(x+1))^2} = \frac{4(x+1) - 2(2x + 1)}{4(x+1)^2} = \frac{4x + 4 - 4x - 2}{4(x+1)^2} = \frac{2}{4(x+1)^2} = \frac{1}{2(x+1)^2}$$.
Step 4: Result matches the chain rule method.
5. Problem: Find $$\frac{dy}{dx}$$ if $$y = \sin^3(4x^2 + 2x)$$.
Step 1: Rewrite as $$y = (\sin(4x^2 + 2x))^3$$.
Step 2: Use chain rule:
$$\frac{dy}{dx} = 3(\sin(4x^2 + 2x))^2 \cdot \cos(4x^2 + 2x) \cdot \frac{d}{dx}(4x^2 + 2x)$$.
Step 3: Compute inner derivative:
$$\frac{d}{dx}(4x^2 + 2x) = 8x + 2$$.
Step 4: Final derivative:
$$\frac{dy}{dx} = 3 \sin^2(4x^2 + 2x) \cdot \cos(4x^2 + 2x) \cdot (8x + 2)$$.
6. Problem: Find the derivative of $$y = \tan 4x^2$$.
Step 1: Use chain rule:
$$\frac{dy}{dx} = \sec^2(4x^2) \cdot \frac{d}{dx}(4x^2) = \sec^2(4x^2) \cdot 8x$$.
7. Problem: Find $$\frac{dy}{dx}$$ if $$y = \sin(\ln x)$$.
Step 1: Use chain rule:
$$\frac{dy}{dx} = \cos(\ln x) \cdot \frac{d}{dx}(\ln x) = \cos(\ln x) \cdot \frac{1}{x} = \frac{\cos(\ln x)}{x}$$.
8. Problem: Find $$\frac{dy}{dx}$$ if $$y = \frac{\sin x}{x}$$.
Step 1: Use quotient rule:
$$\frac{dy}{dx} = \frac{x \cdot \cos x - \sin x \cdot 1}{x^2} = \frac{x \cos x - \sin x}{x^2}$$.
9. Problem: Find the increments.
(i) $$F(x) = 2x^2 + 3x - 5, x=2, \Delta x = 0.5$$.
Step 1: Compute $$F(2) = 2(2)^2 + 3(2) - 5 = 8 + 6 - 5 = 9$$.
Step 2: Compute $$F(2.5) = 2(2.5)^2 + 3(2.5) - 5 = 2(6.25) + 7.5 - 5 = 12.5 + 7.5 - 5 = 15$$.
Step 3: Increment $$\Delta F = F(2.5) - F(2) = 15 - 9 = 6$$.
(ii) $$g(x) = \frac{x^2 - 4}{x - 2}, x=1, \Delta x = 2$$.
Step 1: Compute $$g(1) = \frac{1 - 4}{1 - 2} = \frac{-3}{-1} = 3$$.
Step 2: Compute $$g(3) = \frac{9 - 4}{3 - 2} = \frac{5}{1} = 5$$.
Step 3: Increment $$\Delta g = g(3) - g(1) = 5 - 3 = 2$$.