The diagram shows part of the curve of y=x(x−2)2y = x(x-2)^2, which passes through the point P(1,1)P(1, 1).

  1. (a)

    Show that the equation of OPOP is y=xy = x.

  2. (b)

    Show that OPOP is the normal to the curve at PP.

  3. (c)

    Find the area of the shaded region bounded by the curve and the line OPOP.

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Worked solution

13(a)

1. Calculate the gradient of the line $OP$

The coordinates of OO are (0,0)(0, 0) and PP are (1,1)(1, 1).
Gradient of OP=1−01−0=1OP = \frac{1 - 0}{1 - 0} = 1

2. Form the equation of the line $OP$

Since the line passes through the origin (0,0)(0, 0), the yy-intercept c=0c = 0.
Equation of OPOP is y=1x+0y = 1x + 0
y=xy = x (Shown)

Answer

y=xy = x (Shown)

13(b)

1. Find the derivative of the curve's equation.

y=x(x−2)2=x(x2−4x+4)=x3−4x2+4xy = x(x-2)^2 = x(x^2 - 4x + 4) = x^3 - 4x^2 + 4x
dydx=3x2−8x+4\frac{dy}{dx} = 3x^2 - 8x + 4

2. Calculate the gradient of the tangent at $P(1, 1)$.

At x=1x = 1,
dydx=3(1)2−8(1)+4=−1\frac{dy}{dx} = 3(1)^2 - 8(1) + 4 = -1
The gradient of the tangent at PP is −1-1.

3. Show that $OP$ is perpendicular to the tangent at $P$.

From part (a), the equation of OPOP is y=xy = x, so the gradient of OPOP is 11.
Product of gradients = (−1)×1=−1(-1) \times 1 = -1.
Since the product of the gradients is −1-1, OPOP is perpendicular to the tangent at PP. Therefore, OPOP is the normal to the curve at PP.

Answer

Shown that the product of the gradient of the tangent at PP and the gradient of OPOP is −1-1.

13(c)

1. Set up the integral for the area between the curve and the line

Area=∫01[x(x−2)2−x] dx\text{Area} = \int_{0}^{1} \left[ x(x-2)^2 - x \right] \, dx

2. Expand and simplify the integrand

x(x−2)2−x=x(x2−4x+4)−xx(x-2)^2 - x = x(x^2 - 4x + 4) - x
=x3−4x2+4x−x= x^3 - 4x^2 + 4x - x
=x3−4x2+3x= x^3 - 4x^2 + 3x

3. Integrate the polynomial

∫01(x3−4x2+3x) dx=[x44−4x33+3x22]01\int_{0}^{1} (x^3 - 4x^2 + 3x) \, dx = \left[ \frac{x^4}{4} - \frac{4x^3}{3} + \frac{3x^2}{2} \right]_{0}^{1}

4. Evaluate the definite integral using the limits

=(14−43+32)−(0)= \left( \frac{1}{4} - \frac{4}{3} + \frac{3}{2} \right) - (0)
=312−1612+1812= \frac{3}{12} - \frac{16}{12} + \frac{18}{12}
=512= \frac{5}{12}

Answer

512\frac{5}{12}

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