The curve CC has equation y=x2x−1y = \frac{x^2}{x - 1}. Points A(x1,y1)A(x_1, y_1) and B(x2,y2)B(x_2, y_2) lie on curve CC such that the tangent at AA is parallel to tangent at BB where x2<x1x_2 < x_1. Given further that the equation of tangent at AA is y=89x+169y = \frac{8}{9}x + \frac{16}{9}, find the coordinates of BB, and hence find the equation of normal at point BB.

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Worked solution

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1. Find the derivative of the curve equation.

y=x2x−1y = \frac{x^2}{x-1}
Using the quotient rule:
dydx=(x−1)(2x)−x2(1)(x−1)2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{(x-1)(2x) - x^2(1)}{(x-1)^2}
dydx=2x2−2x−x2(x−1)2=x2−2x(x−1)2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{2x^2 - 2x - x^2}{(x-1)^2} = \frac{x^2 - 2x}{(x-1)^2}

2. Equate the derivative to the gradient of the given tangent to solve for possible $x$-coordinates.

The equation of tangent at AA is y=89x+169y = \frac{8}{9}x + \frac{16}{9}, so its gradient is 89\frac{8}{9}.
Since the tangent at BB is parallel to the tangent at AA, both have a gradient of 89\frac{8}{9}.
Equating dydx\frac{\mathrm{d}y}{\mathrm{d}x} to 89\frac{8}{9}:
x2−2x(x−1)2=89\frac{x^2 - 2x}{(x-1)^2} = \frac{8}{9}
9(x2−2x)=8(x2−2x+1)9(x^2 - 2x) = 8(x^2 - 2x + 1)
9x2−18x=8x2−16x+89x^2 - 18x = 8x^2 - 16x + 8
x2−2x−8=0x^2 - 2x - 8 = 0

3. Solve for the coordinates of $B$.

(x−4)(x+2)=0(x-4)(x+2) = 0
x=4 or x=−2x = 4 \text{ or } x = -2
Since x2<x1x_2 < x_1, x2=−2x_2 = -2 and x1=4x_1 = 4.
Substitute x=−2x = -2 into the curve equation to find y2y_2:
y2=(−2)2−2−1=4−3=−43y_2 = \frac{(-2)^2}{-2 - 1} = \frac{4}{-3} = -\frac{4}{3}
Coordinates of BB are (−2,−43)\left(-2, -\frac{4}{3}\right).

4. Find the gradient of the normal at $B$.

Gradient of tangent at BB, mT=89m_T = \frac{8}{9}.
Gradient of normal at BB, mN=−1mT=−98m_N = -\frac{1}{m_T} = -\frac{9}{8}.

5. Form the equation of the normal at $B$.

Using the point-gradient form y−y1=m(x−x1)y - y_1 = m(x - x_1):
y−(−43)=−98(x−(−2))y - \left(-\frac{4}{3}\right) = -\frac{9}{8}(x - (-2))
y+43=−98x−94y + \frac{4}{3} = -\frac{9}{8}x - \frac{9}{4}
y=−98x−94−43y = -\frac{9}{8}x - \frac{9}{4} - \frac{4}{3}
y=−98x−4312y = -\frac{9}{8}x - \frac{43}{12}

Answer

Coordinates of BB are (−2,−43)\left(-2, -\frac{4}{3}\right). Equation of normal is y=−98x−4312y = -\frac{9}{8}x - \frac{43}{12}.

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