Show that for x>1x>1,

  1. ((i))

    ddx(1x2−1)=−x(x2−1)3/2 .\dfrac{\mathrm{d}}{\mathrm{d}x}\left(\dfrac{1}{\sqrt{x^2-1}}\right)=-\dfrac{x}{(x^2-1)^{3/2}}\,.

  2. ((ii))

    ddx(sin⁡−11x)=−1xx2−1 .\dfrac{\mathrm{d}}{\mathrm{d}x}\left(\sin^{-1}\dfrac{1}{x}\right)=-\dfrac{1}{x\sqrt{x^2-1}}\,.

  3. A curve CC has parametric equations

    x=1t2−1,y=ln⁡twhere t>1.x=\dfrac{1}{\sqrt{t^2-1}},\quad y=\ln t\quad\text{where }t>1.

    RR is the finite region bounded by the curve CC, the xx-axis and the lines x=13x=\dfrac{1}{\sqrt{3}} and x=3x=\sqrt{3}.

    Show that the area of RR is given by

    ∫232t(t2−1)3/2(ln⁡t) dt .\displaystyle\int_{\frac{2}{\sqrt{3}}}^{2}\dfrac{t}{(t^2-1)^{3/2}}(\ln t)\,\mathrm{d}t\,.

  4. By using the results in (i) & (ii), find the exact area of RR.

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Worked solution

9((i))

1. Express the given function with a negative rational index and apply the chain rule.

ddx(1x2−1)=ddx((x2−1)−12)=−12(x2−1)−32⋅ddx(x2−1)\begin{aligned}\dfrac{\mathrm{d}}{\mathrm{d}x}\left(\dfrac{1}{\sqrt{x^2-1}}\right) &= \dfrac{\mathrm{d}}{\mathrm{d}x}\left((x^2-1)^{-\frac{1}{2}}\right) \\ &= -\frac{1}{2}(x^2-1)^{-\frac{3}{2}} \cdot \dfrac{\mathrm{d}}{\mathrm{d}x}(x^2-1) \end{aligned}

2. Evaluate the inner derivative and simplify the expression to the required form.

−12(x2−1)−32⋅(2x)=−x(x2−1)−32=−x(x2−1)32\begin{aligned} -\frac{1}{2}(x^2-1)^{-\frac{3}{2}} \cdot (2x) &= -x(x^2-1)^{-\frac{3}{2}} \\ &= -\dfrac{x}{(x^2-1)^{\frac{3}{2}}} \end{aligned}

Answer

-\dfrac{x}{(x^2-1)^{3/2}} \quad \text{(shown)}

9((ii))

1. Apply the chain rule using the standard derivative for the inverse sine function.

ddx(sin⁡−11x)=11−(1x)2⋅ddx(x−1)\dfrac{\mathrm{d}}{\mathrm{d}x}\left(\sin^{-1}\dfrac{1}{x}\right) = \dfrac{1}{\sqrt{1 - \left(\frac{1}{x}\right)^2}} \cdot \dfrac{\mathrm{d}}{\mathrm{d}x}\left(x^{-1}\right)

2. Simplify the expression algebraically, taking careful note of the condition $x > 1$ when extracting the square root.

=1x2−1x2⋅(−x−2)= \dfrac{1}{\sqrt{\dfrac{x^2 - 1}{x^2}}} \cdot \left(-x^{-2}\right)
=1x2−1∣x∣⋅(−1x2)= \dfrac{1}{\dfrac{\sqrt{x^2 - 1}}{|x|}} \cdot \left(-\dfrac{1}{x^2}\right)
Since it is given that x>1x > 1, we have ∣x∣=x|x| = x. Thus:
=xx2−1⋅(−1x2)= \dfrac{x}{\sqrt{x^2 - 1}} \cdot \left(-\dfrac{1}{x^2}\right)
=−1xx2−1(shown)= -\dfrac{1}{x\sqrt{x^2-1}} \quad \text{(shown)}

Answer

\text{Shown}

9[2]

1. Find the limits of the parameter $t$ corresponding to the boundary lines $x = \frac{1}{\sqrt{3}}$ and $x = \sqrt{3}$.

When x=13x = \frac{1}{\sqrt{3}},
1t2−1=13\frac{1}{\sqrt{t^2-1}} = \frac{1}{\sqrt{3}}
t2−1=3  ⟹  t2=4  ⟹  t=2(since t>1)t^2 - 1 = 3 \implies t^2 = 4 \implies t = 2 \quad (\text{since } t > 1)

When x=3x = \sqrt{3},
1t2−1=3\frac{1}{\sqrt{t^2-1}} = \sqrt{3}
t2−1=13  ⟹  t2=43  ⟹  t=23(since t>1)t^2 - 1 = \frac{1}{3} \implies t^2 = \frac{4}{3} \implies t = \frac{2}{\sqrt{3}} \quad (\text{since } t > 1)

2. Determine $\frac{\mathrm{d}x}{\mathrm{d}t}$ using the result from part (i).

From part (i), we are given ddx(1x2−1)=−x(x2−1)3/2\frac{\mathrm{d}}{\mathrm{d}x}\left(\frac{1}{\sqrt{x^2-1}}\right)=-\frac{x}{(x^2-1)^{3/2}}.
Replacing xx with tt, we obtain:
dxdt=−t(t2−1)3/2\frac{\mathrm{d}x}{\mathrm{d}t} = -\frac{t}{(t^2-1)^{3/2}}

3. Set up the definite integral for the area and reverse the limits to match the required form.

Area=∫133y dx\text{Area} = \int_{\frac{1}{\sqrt{3}}}^{\sqrt{3}} y \,\mathrm{d}x
Substituting y=ln⁡ty = \ln t, dx=dxdtdt\mathrm{d}x = \frac{\mathrm{d}x}{\mathrm{d}t}\mathrm{d}t and the calculated tt limits:
=∫223(ln⁡t)(−t(t2−1)3/2) dt= \int_{2}^{\frac{2}{\sqrt{3}}} (\ln t) \left( -\frac{t}{(t^2-1)^{3/2}} \right) \,\mathrm{d}t
Reversing the limits of integration changes the sign of the integral:
=∫232t(t2−1)3/2(ln⁡t) dt(Shown)= \int_{\frac{2}{\sqrt{3}}}^{2} \frac{t}{(t^2-1)^{3/2}}(\ln t) \,\mathrm{d}t \quad \text{(Shown)}

Answer

∫232t(t2−1)3/2(ln⁡t) dt(Shown)\int_{\frac{2}{\sqrt{3}}}^{2}\frac{t}{(t^2-1)^{3/2}}(\ln t)\,\mathrm{d}t\quad\text{(Shown)}

9[3]

1. Set up the components for integration by parts.

Let u=ln⁡t  ⟹  dudt=1tu = \ln t \implies \dfrac{\mathrm{d}u}{\mathrm{d}t} = \dfrac{1}{t}

Let dvdt=t(t2−1)3/2\dfrac{\mathrm{d}v}{\mathrm{d}t} = \dfrac{t}{(t^2-1)^{3/2}}

From the result in (i), v=−1t2−1v = -\dfrac{1}{\sqrt{t^2-1}}

2. Apply the integration by parts formula.

Using the formula ∫udvdt dt=uv−∫vdudt dt\int u \dfrac{\mathrm{d}v}{\mathrm{d}t} \,\mathrm{d}t = uv - \int v \dfrac{\mathrm{d}u}{\mathrm{d}t} \,\mathrm{d}t:

Area=[−ln⁡tt2−1]232−∫232(−1t2−1)(1t) dt\text{Area} = \left[ -\dfrac{\ln t}{\sqrt{t^2-1}} \right]_{\frac{2}{\sqrt{3}}}^{2} - \int_{\frac{2}{\sqrt{3}}}^{2} \left( -\dfrac{1}{\sqrt{t^2-1}} \right) \left( \dfrac{1}{t} \right) \,\mathrm{d}t

Area=[−ln⁡tt2−1]232+∫2321tt2−1 dt\text{Area} = \left[ -\dfrac{\ln t}{\sqrt{t^2-1}} \right]_{\frac{2}{\sqrt{3}}}^{2} + \int_{\frac{2}{\sqrt{3}}}^{2} \dfrac{1}{t\sqrt{t^2-1}} \,\mathrm{d}t

3. Evaluate the remaining integral using the result from (ii).

From the result in (ii), we know that ∫1tt2−1 dt=−sin⁡−1(1t)\int \dfrac{1}{t\sqrt{t^2-1}} \,\mathrm{d}t = -\sin^{-1}\left(\dfrac{1}{t}\right)

Area=[−ln⁡tt2−1−sin⁡−1(1t)]232\text{Area} = \left[ -\dfrac{\ln t}{\sqrt{t^2-1}} - \sin^{-1} \left( \dfrac{1}{t} \right) \right]_{\frac{2}{\sqrt{3}}}^{2}

4. Substitute the upper limit of integration.

Substitute t=2t=2 into the expression:

−ln⁡222−1−sin⁡−1(12)=−ln⁡23−π6-\dfrac{\ln 2}{\sqrt{2^2-1}} - \sin^{-1} \left( \dfrac{1}{2} \right) = -\dfrac{\ln 2}{\sqrt{3}} - \dfrac{\pi}{6}

5. Substitute the lower limit of integration.

Substitute t=23t=\dfrac{2}{\sqrt{3}} into the expression:

−ln⁡(23)43−1−sin⁡−1(32)-\dfrac{\ln \left( \frac{2}{\sqrt{3}} \right)}{\sqrt{\frac{4}{3}-1}} - \sin^{-1} \left( \dfrac{\sqrt{3}}{2} \right)

=−ln⁡2−12ln⁡313−π3= -\dfrac{\ln 2 - \frac{1}{2}\ln 3}{\frac{1}{\sqrt{3}}} - \dfrac{\pi}{3}

=−3ln⁡2+32ln⁡3−π3= -\sqrt{3}\ln 2 + \dfrac{\sqrt{3}}{2}\ln 3 - \dfrac{\pi}{3}

6. Calculate the exact final area by finding the difference.

Area=(−33ln⁡2−π6)−(−3ln⁡2+32ln⁡3−π3)\text{Area} = \left( -\dfrac{\sqrt{3}}{3}\ln 2 - \dfrac{\pi}{6} \right) - \left( -\sqrt{3}\ln 2 + \dfrac{\sqrt{3}}{2}\ln 3 - \dfrac{\pi}{3} \right)

=−33ln⁡2+3ln⁡2−32ln⁡3−π6+π3= -\dfrac{\sqrt{3}}{3}\ln 2 + \sqrt{3}\ln 2 - \dfrac{\sqrt{3}}{2}\ln 3 - \dfrac{\pi}{6} + \dfrac{\pi}{3}

=233ln⁡2−32ln⁡3+π6= \dfrac{2\sqrt{3}}{3}\ln 2 - \dfrac{\sqrt{3}}{2}\ln 3 + \dfrac{\pi}{6}

Answer

233ln⁡2−32ln⁡3+π6\frac{2\sqrt{3}}{3} \ln 2 - \frac{\sqrt{3}}{2} \ln 3 + \frac{\pi}{6}

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