Solve the equation

  1. (i)

    2log⁡3(x+4)−log⁡3(x+2)=22\log_3(x + 4) - \log_3(x + 2) = 2,

  2. (ii)

    log⁡5y−3log⁡y5=2\log_5 y - 3\log_y 5 = 2.

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Worked solution

6(i)

1. Apply the power law of logarithms

log⁡3(x+4)2−log⁡3(x+2)=2\log_3(x + 4)^2 - \log_3(x + 2) = 2

2. Apply the quotient law of logarithms

log⁡3(x+4)2x+2=2\log_3 \frac{(x + 4)^2}{x + 2} = 2

3. Convert the logarithmic equation to exponential form

(x+4)2x+2=32\frac{(x + 4)^2}{x + 2} = 3^2
(x+4)2x+2=9\frac{(x + 4)^2}{x + 2} = 9

4. Form and solve the resulting quadratic equation

(x+4)2=9(x+2)(x + 4)^2 = 9(x + 2)
x2+8x+16=9x+18x^2 + 8x + 16 = 9x + 18
x2−x−2=0x^2 - x - 2 = 0
(x−2)(x+1)=0(x - 2)(x + 1) = 0
x=2 or x=−1x = 2 \text{ or } x = -1

5. Check the validity of the solutions

For x=2x = 2, x+4=6>0x + 4 = 6 > 0 and x+2=4>0x + 2 = 4 > 0.
For x=−1x = -1, x+4=3>0x + 4 = 3 > 0 and x+2=1>0x + 2 = 1 > 0.
Both solutions are valid.

Answer

x=2x = 2 or x=−1x = -1

6(ii)

1. Apply the change of base formula to express all logarithms in the same base.

log⁡5y−3(1log⁡5y)=2\log_5 y - 3\left(\frac{1}{\log_5 y}\right) = 2

2. Use substitution to form a quadratic equation.

Let u=log⁡5yu = \log_5 y.
u−3u=2u - \frac{3}{u} = 2
u2−2u−3=0u^2 - 2u - 3 = 0

3. Solve the quadratic equation for $u$.

(u−3)(u+1)=0(u - 3)(u + 1) = 0
u=3u = 3 or u=−1u = -1

4. Solve for $y$ and verify the solutions.

When u=3u = 3:
log⁡5y=3  ⟹  y=53=125\log_5 y = 3 \implies y = 5^3 = 125

When u=−1u = -1:
log⁡5y=−1  ⟹  y=5−1=15\log_5 y = -1 \implies y = 5^{-1} = \frac{1}{5}

Both y=125y = 125 and y=15y = \frac{1}{5} are valid since y>0y > 0 and y≠1y \neq 1.

Answer

y=125y = 125 or y=15y = \frac{1}{5}

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