The diagram shows part of the curve y=ex−1−2y = e^{x-1} - 2. The point P(2, e−2)P(2,\ e-2) lies on the curve and the normal to the curve at PP meets the x-axis at QQ.

  1. (a)

    Find the x-coordinate of QQ in terms of ee.

  2. (b)

    Find the total area of the shaded region.

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Worked solution

10(a)

1. Differentiate the equation of the curve to find the gradient function

y=ex−1−2y = e^{x-1} - 2
dydx=ex−1\frac{dy}{dx} = e^{x-1}

2. Find the gradient of the tangent at $P(2, e-2)$

At x=2x = 2, dydx=e2−1=e\frac{dy}{dx} = e^{2-1} = e

3. Find the gradient of the normal at $P$

Gradient of normal mn=−1mt=−1em_n = -\frac{1}{m_t} = -\frac{1}{e}

4. Form the equation of the normal at $P$

y−(e−2)=−1e(x−2)y - (e-2) = -\frac{1}{e}(x - 2)

5. Find the x-coordinate of $Q$ by setting $y = 0$

At QQ, y=0y = 0:
0−(e−2)=−1e(x−2)0 - (e-2) = -\frac{1}{e}(x - 2)
−(e−2)=−1e(x−2)-(e-2) = -\frac{1}{e}(x - 2)
e(e−2)=x−2e(e-2) = x - 2
x=2+e(e−2)x = 2 + e(e-2)

Answer

x=2+e(e−2)x = 2 + e(e-2)

10(b)

1. Find the x-intercept of the curve to determine the limits of integration.

At the x-axis, y=0y = 0:
ex−1−2=0e^{x-1} - 2 = 0
ex−1=2e^{x-1} = 2
x−1=ln⁡2x - 1 = \ln 2
x=1+ln⁡2x = 1 + \ln 2

2. Set up the definite integral for the area of the shaded region.

The shaded region is bounded by the curve, the x-axis, and the y-axis. Since the curve is below the x-axis for 0⩽x⩽1+ln⁡20 \leqslant x \leqslant 1 + \ln 2, the area is:
Area=−∫01+ln⁡2(ex−1−2) dx\text{Area} = -\int_{0}^{1+\ln 2} (e^{x-1} - 2) \, dx

3. Integrate the function.

∫(ex−1−2) dx=ex−1−2x\int (e^{x-1} - 2) \, dx = e^{x-1} - 2x

4. Evaluate the definite integral using the limits.

Area=−[ex−1−2x]01+ln⁡2\text{Area} = - \left[ e^{x-1} - 2x \right]_{0}^{1+\ln 2}
=−((e1+ln⁡2−1−2(1+ln⁡2))−(e−1−0))= - \left( (e^{1+\ln 2 - 1} - 2(1+\ln 2)) - (e^{-1} - 0) \right)
=−(eln⁡2−2−2ln⁡2−1e)= - \left( e^{\ln 2} - 2 - 2\ln 2 - \frac{1}{e} \right)
=−(2−2−2ln⁡2−1e)= - \left( 2 - 2 - 2\ln 2 - \frac{1}{e} \right)
=2ln⁡2+1e= 2\ln 2 + \frac{1}{e}

Answer

2ln⁡2+1e2\ln 2 + \frac{1}{e}

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