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$$ \text{Let } f \text{ be a differentiable function such that } 2(x+2)^2f(x)-3(x+2)^2 = 10\int_0^x (t+2)f(t)\,dt,\quad x\ge0. \text{ Then } f(2) \text{ is equal to:}\underline{\hspace{1cm}} $$
Correct Answer: 19
Given $$2(x+2)^2f(x) - 3(x+2)^2 = 10\int_0^x(t+2)f(t)dt$$, find f(2).
Differentiate both sides with respect to x
$$4(x+2)f(x) + 2(x+2)^2f'(x) - 6(x+2) = 10(x+2)f(x)$$
$$2(x+2)^2f'(x) = 6(x+2)f(x) + 6(x+2)$$
$$2(x+2)f'(x) = 6f(x) + 6$$
$$(x+2)f'(x) - 3f(x) = 3$$
Find f(0) from original equation
At x=0: $$2(4)f(0) - 3(4) = 0$$
$$8f(0) = 12 \Rightarrow f(0) = 3/2$$
Solve the ODE
$$f'(x) - \frac{3}{x+2}f(x) = \frac{3}{x+2}$$
IF = $$e^{-3\ln(x+2)} = \frac{1}{(x+2)^3}$$
$$\frac{d}{dx}\left[\frac{f(x)}{(x+2)^3}\right] = \frac{3}{(x+2)^4}$$
$$\frac{f(x)}{(x+2)^3} = -\frac{1}{(x+2)^3} + C$$
$$f(x) = -1 + C(x+2)^3$$
Apply f(0) = 3/2
$$3/2 = -1 + C(8) \Rightarrow C = \frac{5/2}{8} = \frac{5}{16}$$
$$f(x) = -1 + \frac{5}{16}(x+2)^3$$
Find f(2)
$$f(2) = -1 + \frac{5}{16}(4)^3 = -1 + \frac{5 \times 64}{16} = -1 + 20 = 19$$
The answer is 19.
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