Given that 2x−2×3x+2=62x2^{x-2} \times 3^{x+2} = 6^{2x}, show that x=lg⁡9−lg⁡4lg⁡6x = \frac{\lg 9 - \lg 4}{\lg 6}. Solve log⁡9y−2=log⁡32y\log_9 y - 2 = \log_3 2y.

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Worked solution

12(a)

1. Take logarithms to base 10 on both sides of the equation.

lg⁡(2x−2×3x+2)=lg⁡(62x)\lg(2^{x-2} \times 3^{x+2}) = \lg(6^{2x})

2. Apply the product and power laws of logarithms to expand the equation.

lg⁡(2x−2)+lg⁡(3x+2)=2xlg⁡6\lg(2^{x-2}) + \lg(3^{x+2}) = 2x \lg 6
(x−2)lg⁡2+(x+2)lg⁡3=2xlg⁡6(x-2)\lg 2 + (x+2)\lg 3 = 2x \lg 6

3. Expand the terms and group the terms involving $x$.

xlg⁡2−2lg⁡2+xlg⁡3+2lg⁡3=2xlg⁡6x \lg 2 - 2 \lg 2 + x \lg 3 + 2 \lg 3 = 2x \lg 6
x(lg⁡2+lg⁡3)−lg⁡(22)+lg⁡(32)=2xlg⁡6x(\lg 2 + \lg 3) - \lg(2^2) + \lg(3^2) = 2x \lg 6
xlg⁡(2×3)−lg⁡4+lg⁡9=2xlg⁡6x \lg(2 \times 3) - \lg 4 + \lg 9 = 2x \lg 6
xlg⁡6−lg⁡4+lg⁡9=2xlg⁡6x \lg 6 - \lg 4 + \lg 9 = 2x \lg 6

4. Rearrange to make $x$ the subject.

lg⁡9−lg⁡4=2xlg⁡6−xlg⁡6\lg 9 - \lg 4 = 2x \lg 6 - x \lg 6
lg⁡9−lg⁡4=xlg⁡6\lg 9 - \lg 4 = x \lg 6
x=lg⁡9−lg⁡4lg⁡6x = \frac{\lg 9 - \lg 4}{\lg 6}

Answer

Shown

12(b)

1. Change the base of $\log_9 y$ to base 3

log⁡9y=log⁡3ylog⁡39=log⁡3y2\log_9 y = \frac{\log_3 y}{\log_3 9} = \frac{\log_3 y}{2}

2. Substitute into the equation and eliminate fractions

12log⁡3y−2=log⁡32y\frac{1}{2}\log_3 y - 2 = \log_3 2y
log⁡3y−4=2log⁡32y\log_3 y - 4 = 2\log_3 2y

3. Apply power and quotient laws of logarithms to combine terms

log⁡3y−4=log⁡3(2y)2\log_3 y - 4 = \log_3 (2y)^2
log⁡3y−4=log⁡3(4y2)\log_3 y - 4 = \log_3 (4y^2)
−4=log⁡3(4y2)−log⁡3y-4 = \log_3 (4y^2) - \log_3 y
−4=log⁡3(4y2y)-4 = \log_3 \left(\frac{4y^2}{y}\right)
−4=log⁡3(4y)-4 = \log_3 (4y)

4. Convert to exponential form and solve for $y$

4y=3−44y = 3^{-4}
4y=1814y = \frac{1}{81}
y=1324y = \frac{1}{324}

Answer

y=1324y = \frac{1}{324}

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