The equation of a curve is y=xe−3xy = xe^{-3x}. Find the stationary point(s) and determine the nature of the stationary point(s).

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Worked solution

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1. Find the first derivative $\frac{dy}{dx}$ using the product rule.

y=xe−3xy = xe^{-3x}

Let u=xu = x and v=e−3xv = e^{-3x}.
dudx=1\frac{du}{dx} = 1
dvdx=−3e−3x\frac{dv}{dx} = -3e^{-3x}

dydx=udvdx+vdudx\frac{dy}{dx} = u\frac{dv}{dx} + v\frac{du}{dx}
dydx=x(−3e−3x)+e−3x(1)\frac{dy}{dx} = x(-3e^{-3x}) + e^{-3x}(1)
dydx=e−3x(1−3x)\frac{dy}{dx} = e^{-3x}(1 - 3x)

2. Equate the first derivative to zero to find the $x$-coordinate of the stationary point.

For stationary points, dydx=0\frac{dy}{dx} = 0.
e−3x(1−3x)=0e^{-3x}(1 - 3x) = 0

Since e−3x>0e^{-3x} > 0 for all real xx,
1−3x=01 - 3x = 0
3x=13x = 1
x=13x = \frac{1}{3}

3. Find the corresponding $y$-coordinate.

When x=13x = \frac{1}{3},
y=(13)e−3(13)y = \left(\frac{1}{3}\right)e^{-3(\frac{1}{3})}
y=13e−1=13ey = \frac{1}{3}e^{-1} = \frac{1}{3e}

The stationary point is (13,13e)\left(\frac{1}{3}, \frac{1}{3e}\right).

4. Find the second derivative to determine the nature of the stationary point.

dydx=e−3x(1−3x)\frac{dy}{dx} = e^{-3x}(1 - 3x)

Using the product rule again:
d2ydx2=e−3x(−3)+(1−3x)(−3e−3x)\frac{d^2y}{dx^2} = e^{-3x}(-3) + (1 - 3x)(-3e^{-3x})
d2ydx2=−3e−3x−3e−3x+9xe−3x\frac{d^2y}{dx^2} = -3e^{-3x} - 3e^{-3x} + 9xe^{-3x}
d2ydx2=e−3x(9x−6)\frac{d^2y}{dx^2} = e^{-3x}(9x - 6)

5. Evaluate the second derivative at the stationary point to conclude its nature.

When x=13x = \frac{1}{3},
d2ydx2=e−1(9(13)−6)\frac{d^2y}{dx^2} = e^{-1}\left(9\left(\frac{1}{3}\right) - 6\right)
d2ydx2=e−1(3−6)=−3e−1\frac{d^2y}{dx^2} = e^{-1}(3 - 6) = -3e^{-1}

Since e−1>0e^{-1} > 0, d2ydx2<0\frac{d^2y}{dx^2} < 0.
Therefore, the stationary point is a maximum point.

Answer

Stationary point is (13,13e)\left(\frac{1}{3}, \frac{1}{3e}\right), which is a maximum point.

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