Show that the solution of the equation 32x−1=23−x3^{2x-1} = 2^{3-x} is x=lg⁡24lg⁡18x = \frac{\lg 24}{\lg 18}. Given that log⁡64y=log⁡63x2−1\log_6 4y = \log_6 3x^2 - 1, express yy in terms of xx. Solve the equation log⁡2x2=8log⁡x2\log_2 x^2 = 8\log_x 2.

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Worked solution

12(a)

1. Take the common logarithm (lg) of both sides of the equation

32x−1=23−x3^{2x-1} = 2^{3-x}
lg⁡(32x−1)=lg⁡(23−x)\lg(3^{2x-1}) = \lg(2^{3-x})

2. Apply the power law of logarithms to bring down the exponents

(2x−1)lg⁡3=(3−x)lg⁡2(2x - 1)\lg 3 = (3 - x)\lg 2

3. Expand both sides to isolate terms with x

2xlg⁡3−lg⁡3=3lg⁡2−xlg⁡22x\lg 3 - \lg 3 = 3\lg 2 - x\lg 2
2xlg⁡3+xlg⁡2=3lg⁡2+lg⁡32x\lg 3 + x\lg 2 = 3\lg 2 + \lg 3

4. Factorise x and apply logarithm laws to simplify the constants

x(2lg⁡3+lg⁡2)=lg⁡(23)+lg⁡3x(2\lg 3 + \lg 2) = \lg(2^3) + \lg 3
x(lg⁡(32)+lg⁡2)=lg⁡8+lg⁡3x(\lg(3^2) + \lg 2) = \lg 8 + \lg 3
x(lg⁡9+lg⁡2)=lg⁡(8×3)x(\lg 9 + \lg 2) = \lg(8 \times 3)
x(lg⁡18)=lg⁡24x(\lg 18) = \lg 24

5. Solve for x to obtain the final expression

x=lg⁡24lg⁡18x = \frac{\lg 24}{\lg 18}

Answer

Shown

12(b)

1. Express the constant 1 as a logarithm with base 6

log⁡64y=log⁡63x2−log⁡66\log_6 4y = \log_6 3x^2 - \log_6 6

2. Apply the quotient law of logarithms to combine the terms on the right hand side

log⁡64y=log⁡6(3x26)\log_6 4y = \log_6 \left(\frac{3x^2}{6}\right)
log⁡64y=log⁡6(x22)\log_6 4y = \log_6 \left(\frac{x^2}{2}\right)

3. Equate the arguments of the logarithms and solve for $y$

4y=x224y = \frac{x^2}{2}
y=x28y = \frac{x^2}{8}

Answer

y=x28y = \frac{x^2}{8}

12(c)

1. Apply the power law and change of base formula to express the equation in terms of $\log_2 x$

log⁡2x2=8log⁡x2\log_2 x^2 = 8\log_x 2
2log⁡2x=8(1log⁡2x)2\log_2 x = 8 \left( \frac{1}{\log_2 x} \right)

2. Form and solve a quadratic equation in $\log_2 x$

Let u=log⁡2xu = \log_2 x.
2u=8u2u = \frac{8}{u}
2u2=82u^2 = 8
u2=4u^2 = 4
u=2 or u=−2u = 2 \text{ or } u = -2

3. Solve for $x$ and check validity

log⁡2x=2  ⟹  x=22=4\log_2 x = 2 \implies x = 2^2 = 4
log⁡2x=−2  ⟹  x=2−2=14\log_2 x = -2 \implies x = 2^{-2} = \frac{1}{4}
Both values are valid since x>0x > 0 and x≠1x \neq 1.

Answer

x=4x = 4 or x=14x = \frac{1}{4}

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