The solution of the equation 32x−1=4(3x)3^{2x-1}=4(3^x) can be expressed as ln⁡pln⁡q\dfrac{\ln p}{\ln q}.

  1. (a)

    Find the value of pp and of qq.

  2. (b)

    Express the solution of log⁡16(3y−1)+12=log⁡4(3y)\log_{16}(3y-1)+\tfrac{1}{2}=\log_{4}(3y) as a rational number.

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Worked solution

10(a)

1. Rewrite the equation using laws of indices to isolate terms in $3^x$

32x⋅3−1=4(3x)3^{2x} \cdot 3^{-1} = 4(3^x)
13(3x)2=4(3x)\frac{1}{3}(3^x)^2 = 4(3^x)

2. Use substitution to form a quadratic equation

Let u=3xu = 3^x.
13u2=4u\frac{1}{3}u^2 = 4u
u2−12u=0u^2 - 12u = 0

3. Solve the quadratic equation for $u$ and reject the invalid solution

u(u−12)=0u(u - 12) = 0
u=0 or u=12u = 0 \text{ or } u = 12
Since u=3x>0u = 3^x > 0, u=0u = 0 has no solution.
3x=123^x = 12

4. Solve for $x$ using logarithms and identify $p$ and $q$

ln⁡(3x)=ln⁡12\ln(3^x) = \ln 12
xln⁡3=ln⁡12x \ln 3 = \ln 12
x=ln⁡12ln⁡3x = \frac{\ln 12}{\ln 3}
Comparing with ln⁡pln⁡q\frac{\ln p}{\ln q}, we have p=12p = 12 and q=3q = 3.

Answer

p=12p = 12, q=3q = 3

10(b)

1. Change the base of the logarithm from 16 to 4.

log⁡16(3y−1)=log⁡4(3y−1)log⁡416=log⁡4(3y−1)2\log_{16}(3y-1) = \frac{\log_4(3y-1)}{\log_4 16} = \frac{\log_4(3y-1)}{2}

2. Substitute back into the equation and clear the denominator.

12log⁡4(3y−1)+12=log⁡4(3y)\frac{1}{2}\log_4(3y-1) + \frac{1}{2} = \log_4(3y)
log⁡4(3y−1)+1=2log⁡4(3y)\log_4(3y-1) + 1 = 2\log_4(3y)

3. Use laws of logarithms to combine terms into a single equation without logarithms.

log⁡4(3y−1)+log⁡44=log⁡4((3y)2)\log_4(3y-1) + \log_4 4 = \log_4((3y)^2)
log⁡4(4(3y−1))=log⁡4(9y2)\log_4(4(3y-1)) = \log_4(9y^2)
4(3y−1)=9y24(3y-1) = 9y^2
12y−4=9y212y - 4 = 9y^2

4. Solve the resulting quadratic equation for $y$.

9y2−12y+4=09y^2 - 12y + 4 = 0
(3y−2)2=0(3y - 2)^2 = 0
3y−2=03y - 2 = 0
y=23y = \frac{2}{3}

Answer

y=23y = \frac{2}{3}

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