Given that log⁡a8+3log⁡aN=3\log_a 8 + 3\log_a N = 3 and N>14N > \frac{1}{4}, find the range of values of aa. Solve the equation log⁡3x−log⁡x3x−2=0\log_3 x - \log_x 3x - 2 = 0.

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Worked solution

4(a)

1. Combine the logarithmic terms into a single logarithm.

log⁡a8+log⁡aN3=3\log_a 8 + \log_a N^3 = 3
log⁡a(8N3)=3\log_a (8N^3) = 3

2. Convert the logarithmic equation to exponential form and solve for $a$ in terms of $N$.

8N3=a38N^3 = a^3
(2N)3=a3(2N)^3 = a^3
2N=a2N = a
N=a2N = \frac{a}{2}

3. Use the given inequality for $N$ to find the inequality for $a$.

Since N>14N > \frac{1}{4},
a2>14\frac{a}{2} > \frac{1}{4}
a>12a > \frac{1}{2}

4. Apply the conditions for the base of a logarithm to state the final range of $a$.

Since aa is the base of a logarithm, a>0a > 0 and a≠1a \neq 1.
The condition a>12a > \frac{1}{2} satisfies a>0a > 0, but we must exclude a=1a = 1.
Therefore, a>12,a≠1a > \frac{1}{2}, a \neq 1.

Answer

a>12,a≠1a > \frac{1}{2}, a \neq 1

4(b)

1. Express $\log_x 3x$ in terms of base 3 logarithms

log⁡x3x=log⁡33xlog⁡3x=log⁡33+log⁡3xlog⁡3x=1+log⁡3xlog⁡3x\log_x 3x = \frac{\log_3 3x}{\log_3 x} = \frac{\log_3 3 + \log_3 x}{\log_3 x} = \frac{1 + \log_3 x}{\log_3 x}

2. Substitute back into the equation and form a quadratic equation

log⁡3x−1+log⁡3xlog⁡3x−2=0\log_3 x - \frac{1 + \log_3 x}{\log_3 x} - 2 = 0

Let u=log⁡3xu = \log_3 x:
u−1+uu−2=0u - \frac{1 + u}{u} - 2 = 0
u2−(1+u)−2u=0u^2 - (1 + u) - 2u = 0
u2−3u−1=0u^2 - 3u - 1 = 0

3. Solve the quadratic equation for $u$

u=−(−3)±(−3)2−4(1)(−1)2(1)u = \frac{-(-3) \pm \sqrt{(-3)^2 - 4(1)(-1)}}{2(1)}
u=3±132u = \frac{3 \pm \sqrt{13}}{2}
u≈3.3028u \approx 3.3028 or u≈−0.3028u \approx -0.3028

4. Solve for $x$

log⁡3x=3.3028  ⟹  x=33.3028≈37.7\log_3 x = 3.3028 \implies x = 3^{3.3028} \approx 37.7
log⁡3x=−0.3028  ⟹  x=3−0.3028≈0.717\log_3 x = -0.3028 \implies x = 3^{-0.3028} \approx 0.717

Answer

x=37.7x = 37.7 or x=0.717x = 0.717

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