Find the integer aa and bb which satisfy the equation (18y3)−1÷(4y2)12=2ayb.\left(\frac{1}{8}y^{3}\right)^{-1}\div\left(4y^{2}\right)^{\tfrac{1}{2}}=2^{a}y^{b}. Find the integer aa and bb which satisfy the equation (18y3)−1÷(4y2)12=2ayb.\left(\frac{1}{8}y^{3}\right)^{-1}\div\left(4y^{2}\right)^{\tfrac{1}{2}}=2^{a}y^{b}. Solve the equation log⁡3(x−8)=2−1log⁡x3\log_{3}(x-8)=2-\dfrac{1}{\log_{x}3}. Solve the equation log⁡3(x−8)=2−1log⁡x3\log_{3}(x-8)=2-\dfrac{1}{\log_{x}3}. Solve the equation 4x+1+7(2x)=2.4^{x+1}+7\left(2^{x}\right)=2.

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Worked solution

5(a)

1. Simplify the first term using negative index laws

(18y3)−1=(2−3y3)−1=23y−3=8y−3\left(\frac{1}{8}y^3\right)^{-1} = \left(2^{-3}y^3\right)^{-1} = 2^3 y^{-3} = 8y^{-3}

2. Simplify the second term using fractional index laws

(4y2)12=(22y2)12=21y1=2y\left(4y^2\right)^{\frac{1}{2}} = \left(2^2 y^2\right)^{\frac{1}{2}} = 2^1 y^1 = 2y

3. Perform the division and equate to find a and b

(18y3)−1÷(4y2)12=8y−32y=4y−4=22y−4\left(\frac{1}{8}y^3\right)^{-1} \div \left(4y^2\right)^{\frac{1}{2}} = \frac{8y^{-3}}{2y} = 4y^{-4} = 2^2 y^{-4}
Comparing with 2ayb2^a y^b, we have a=2a = 2 and b=−4b = -4.

Answer

a=2a = 2, b=−4b = -4

5(b)

1. Apply change of base to the term $\frac{1}{\log_{x}3}$

1log⁡x3=log⁡3x\frac{1}{\log_{x}3} = \log_{3}x

The equation becomes:
log⁡3(x−8)=2−log⁡3x\log_{3}(x-8) = 2 - \log_{3}x

2. Group logarithmic terms and apply the product law of logarithms

log⁡3(x−8)+log⁡3x=2\log_{3}(x-8) + \log_{3}x = 2
log⁡3(x(x−8))=2\log_{3}(x(x-8)) = 2

3. Convert the logarithmic equation to an exponential equation

x(x−8)=32x(x-8) = 3^2
x2−8x=9x^2 - 8x = 9

4. Solve the resulting quadratic equation

x2−8x−9=0x^2 - 8x - 9 = 0
(x−9)(x+1)=0(x - 9)(x + 1) = 0
x=9x = 9 or x=−1x = -1

5. Check for valid solutions in the original logarithmic equation

For x=−1x = -1, log⁡3(−1−8)\log_{3}(-1-8) is undefined (argument must be positive).
Also, the base of log⁡x3\log_{x}3 must be positive and not equal to 1.
Therefore, x=−1x = -1 is rejected.
x=9x = 9 is valid.

Answer

x=9x = 9

5(c)

1. Express the equation in terms of $2^x$

4(4x)+7(2x)−2=04(4^x) + 7(2^x) - 2 = 0
4(2x)2+7(2x)−2=04(2^x)^2 + 7(2^x) - 2 = 0

2. Use substitution to form and solve a quadratic equation

Let u=2xu = 2^x.
4u2+7u−2=04u^2 + 7u - 2 = 0
(4u−1)(u+2)=0(4u - 1)(u + 2) = 0
u=14 or u=−2u = \frac{1}{4} \text{ or } u = -2

3. Solve for $x$ and reject invalid solutions

When u=14u = \frac{1}{4}:
2x=142^x = \frac{1}{4}
2x=2−22^x = 2^{-2}
x=−2x = -2

When u=−2u = -2:
2x=−22^x = -2
(No real solution since 2x>02^x > 0 for all real xx)

Answer

x=−2x = -2

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