A particle moves along the curve y=5(3x−1)2y=\dfrac{5}{(3x-1)^2}, where x≠13x \ne \dfrac{1}{3}, in such a way that the x-coordinate is decreasing at a rate of 0.10.1 units per second.

  1. (a)

    Find the rate of change of the y-coordinate when x=1x=1.

  2. (b)

    Find the value of xx when yy increases at the rate of 3125\dfrac{3}{125} units per second.

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Worked solution

6(a)

1. Find the derivative of $y$ with respect to $x$

y=5(3x−1)−2y = 5(3x-1)^{-2}

dydx=5(−2)(3x−1)−3(3)\frac{dy}{dx} = 5(-2)(3x-1)^{-3}(3)
dydx=−30(3x−1)−3=−30(3x−1)3\frac{dy}{dx} = -30(3x-1)^{-3} = -\frac{30}{(3x-1)^3}

2. Evaluate $\frac{dy}{dx}$ at $x=1$

When x=1x = 1,
dydx=−30(3(1)−1)3=−308=−3.75\frac{dy}{dx} = -\frac{30}{(3(1)-1)^3} = -\frac{30}{8} = -3.75

3. Use connected rates of change to find $\frac{dy}{dt}$

Given dxdt=−0.1\frac{dx}{dt} = -0.1 (since xx is decreasing),
dydt=dydx×dxdt\frac{dy}{dt} = \frac{dy}{dx} \times \frac{dx}{dt}
dydt=−3.75×(−0.1)=0.375\frac{dy}{dt} = -3.75 \times (-0.1) = 0.375

Answer

0.3750.375 units per second

6(b)

1. Find the derivative of $y$ with respect to $x$

y=5(3x−1)−2y = 5(3x-1)^{-2}
dydx=5(−2)(3x−1)−3(3)=−30(3x−1)−3\frac{dy}{dx} = 5(-2)(3x-1)^{-3}(3) = -30(3x-1)^{-3}

2. Set up the connected rates of change equation and solve for $x$

Using the chain rule, dydt=dydx×dxdt\frac{dy}{dt} = \frac{dy}{dx} \times \frac{dx}{dt}
3125=−30(3x−1)−3×(−0.1)\frac{3}{125} = -30(3x-1)^{-3} \times (-0.1)
3125=3(3x−1)−3\frac{3}{125} = 3(3x-1)^{-3}
1125=1(3x−1)3\frac{1}{125} = \frac{1}{(3x-1)^3}
(3x−1)3=125(3x-1)^3 = 125
3x−1=53x - 1 = 5
3x=63x = 6
x=2x = 2

Answer

x=2x = 2

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