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The curve C has parametric equations $x = ext{ln}(t + 2),$ $y = rac{1}{(t + 1)},$ $t > -1.$ The finite region R between the curve C and the x-axis, bounded by the lines with equations $x = ext{ln} 2$ and $x = ext{ln} 4$, is shown shaded in Figure 3 - Edexcel - A-Level Maths Pure - Question 8 - 2008 - Paper 8

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Question 8

The-curve-C-has-parametric-equations---$x-=--ext{ln}(t-+-2),$---$y-=--rac{1}{(t-+-1)},$--$t->--1.$-The-finite-region-R-between-the-curve-C-and-the-x-axis,-bounded-by-the-lines-with-equations-$x-=--ext{ln}-2$-and-$x-=--ext{ln}-4$,-is-shown-shaded-in-Figure-3-Edexcel-A-Level Maths Pure-Question 8-2008-Paper 8.png

The curve C has parametric equations $x = ext{ln}(t + 2),$ $y = rac{1}{(t + 1)},$ $t > -1.$ The finite region R between the curve C and the x-axis, bounded by... show full transcript

Worked Solution & Example Answer:The curve C has parametric equations $x = ext{ln}(t + 2),$ $y = rac{1}{(t + 1)},$ $t > -1.$ The finite region R between the curve C and the x-axis, bounded by the lines with equations $x = ext{ln} 2$ and $x = ext{ln} 4$, is shown shaded in Figure 3 - Edexcel - A-Level Maths Pure - Question 8 - 2008 - Paper 8

Step 1

Show that the area of R is given by the integral $$\int_{0}^{2} \frac{1}{(t + 1)(t + 2)} \, dt.$$

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Answer

To find the area of the region R, we need to express the area in terms of the parameter tt. The area can be calculated using the formula:

A=abydx,A = \int_{a}^{b} y \, dx, where y=1(t+1)y = \frac{1}{(t + 1)} and dx=dxdtdtdx = \frac{dx}{dt} \, dt.

Calculating dxdx:

dx=ddt(ln(t+2))=1t+2.dx = \frac{d}{dt}(\text{ln}(t + 2)) = \frac{1}{t + 2}.

Thus, the integral for the area becomes:

A=021(t+1)(t+2)dt.A = \int_{0}^{2} \frac{1}{(t + 1) \cdot (t + 2)} \, dt.

This confirms that the area of R is represented accurately by the given integral.

Step 2

Hence find an exact value for this area.

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Answer

To find the exact area, we can compute the integral:

A=021(t+1)(t+2)dt.A = \int_{0}^{2} \frac{1}{(t + 1)(t + 2)} \, dt.

We can perform partial fraction decomposition:

1(t+1)(t+2)=At+1+Bt+2.\frac{1}{(t + 1)(t + 2)} = \frac{A}{t + 1} + \frac{B}{t + 2}.

By solving:

  1. A(t+2)+B(t+1)=1A(t + 2) + B(t + 1) = 1
  2. Setting t=1t = -1, we find A=1A = 1.
  3. Setting t=2t = -2, we find B=1B = -1.

Thus:

A=02(1t+11t+2)dt=[ln(t+1)ln(t+2)]02.A = \int_{0}^{2} \left( \frac{1}{t + 1} - \frac{1}{t + 2} \right) \, dt = \left[\ln(t + 1) - \ln(t + 2) \right]_{0}^{2}.

Evaluating this gives:

(ln(3)ln(4))(ln(1)ln(2))=ln(3)ln(4)+ln(2)=ln(32).\left( \ln(3) - \ln(4) \right) - \left( \ln(1) - \ln(2) \right) = \ln(3) - \ln(4) + \ln(2) = \ln(\frac{3}{2}).

Step 3

Find a cartesian equation of the curve C, in the form $y = f(x)$.

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Answer

We start with the parametric equations:

  1. x=ln(t+2)x = \text{ln}(t + 2)
  2. y=1t+1y = \frac{1}{t + 1}.

To eliminate tt, we solve the first equation for tt:

t+2=ex    t=ex2.t + 2 = e^x \implies t = e^x - 2.

Now substitute tt from the first equation into the second:

y=1(ex2)+1=1ex1.y = \frac{1}{(e^x - 2) + 1} = \frac{1}{e^x - 1}.

Thus, the cartesian equation of the curve C is:

y=1ex1.y = \frac{1}{e^x - 1}.

Step 4

State the domain of values for x for this curve.

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Answer

For the curve y=1ex1y = \frac{1}{e^x - 1} to be defined, the denominator cannot be zero:

  1. Setting ex1>0    ex>1    x>0.e^x - 1 > 0 \implies e^x > 1 \implies x > 0.

Thus, the domain of values for xx is:

x>0.x > 0.

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