Find the first and second derivatives of the function $y = \frac{1+3x}{3x}(3-x)$.

AnalysisDerivativesDifferentiationCalculusFunctions
2025/5/17

1. Problem Description

Find the first and second derivatives of the function y=1+3x3x(3x)y = \frac{1+3x}{3x}(3-x).

2. Solution Steps

First, simplify the expression for yy:
y=1+3x3x(3x)=(1+3x)(3x)3x=3x+9x3x23x=3+8x3x23x=33x+8x3x3x23x=1x+83xy = \frac{1+3x}{3x}(3-x) = \frac{(1+3x)(3-x)}{3x} = \frac{3 - x + 9x - 3x^2}{3x} = \frac{3 + 8x - 3x^2}{3x} = \frac{3}{3x} + \frac{8x}{3x} - \frac{3x^2}{3x} = \frac{1}{x} + \frac{8}{3} - x
Now, find the first derivative yy':
y=ddx(1x+83x)=ddx(x1)+ddx(83)ddx(x)y' = \frac{d}{dx}(\frac{1}{x} + \frac{8}{3} - x) = \frac{d}{dx}(x^{-1}) + \frac{d}{dx}(\frac{8}{3}) - \frac{d}{dx}(x)
ddx(x1)=1x2=1x2\frac{d}{dx}(x^{-1}) = -1x^{-2} = -\frac{1}{x^2}
ddx(83)=0\frac{d}{dx}(\frac{8}{3}) = 0
ddx(x)=1\frac{d}{dx}(x) = 1
Therefore,
y=1x2+01=1x21y' = -\frac{1}{x^2} + 0 - 1 = -\frac{1}{x^2} - 1
Next, find the second derivative yy'':
y=ddx(1x21)=ddx(x2)ddx(1)y'' = \frac{d}{dx}(-\frac{1}{x^2} - 1) = \frac{d}{dx}(-x^{-2}) - \frac{d}{dx}(1)
ddx(x2)=(2)x3=2x3=2x3\frac{d}{dx}(-x^{-2}) = -(-2)x^{-3} = 2x^{-3} = \frac{2}{x^3}
ddx(1)=0\frac{d}{dx}(1) = 0
Therefore,
y=2x30=2x3y'' = \frac{2}{x^3} - 0 = \frac{2}{x^3}

3. Final Answer

The first derivative is y=1x21y' = -\frac{1}{x^2} - 1.
The second derivative is y=2x3y'' = \frac{2}{x^3}.

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