图中显示了曲线 y=x(x−2)2y = x(x-2)^2 的一部分,该曲线过点 P(1,1)P(1, 1)。

  1. (a)

    证明 OPOP 的方程为 y=xy = x。

  2. (b)

    证明 OPOP 是曲线在点 PP 处的法线。

  3. (c)

    求由曲线与直线 OPOP 所围成的阴影区域的面积。

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完整解法

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)

答案

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.

答案

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}

答案

512\frac{5}{12}

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