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Simplify the following: 1log⁡8(x)−1log⁡7(x)+1log⁡6(x)−1log⁡5(x)+1log⁡4(x)−1log⁡3(x)+1log⁡2(x)\dfrac{1}{\log_{8}(x)} - \dfrac{1}{\log_{7}(x)} + \dfrac{1}{\log_{6}(x)} - \dfrac{1}{\log_{5}(x)} + \dfrac{1}{\log_{4}(x)} - \dfrac{1}{\log_{3}(x)} + \dfrac{1}{\log_{2}(x)}

Solution

✅ Correct Option: 3

Using the change of base formula 1log⁡a(x)=log⁡x(a)\dfrac{1}{\log_a(x)} = \log_x(a):

1log⁡8(x)=log⁡x(8)\dfrac{1}{\log_8(x)} = \log_x(8)

1log⁡7(x)=log⁡x(7)\dfrac{1}{\log_7(x)} = \log_x(7)

1log⁡6(x)=log⁡x(6)\dfrac{1}{\log_6(x)} = \log_x(6)

1log⁡5(x)=log⁡x(5)\dfrac{1}{\log_5(x)} = \log_x(5)

1log⁡4(x)=log⁡x(4)\dfrac{1}{\log_4(x)} = \log_x(4)

1log⁡3(x)=log⁡x(3)\dfrac{1}{\log_3(x)} = \log_x(3)

1log⁡2(x)=log⁡x(2)\dfrac{1}{\log_2(x)} = \log_x(2)


The expression becomes:

log⁡x(8)−log⁡x(7)+log⁡x(6)−log⁡x(5)+log⁡x(4)−log⁡x(3)+log⁡x(2)\log_x(8) - \log_x(7) + \log_x(6) - \log_x(5) + \log_x(4) - \log_x(3) + \log_x(2)


Applying logarithm properties log⁡a(m)+log⁡a(n)=log⁡a(mn)\log_a(m) + \log_a(n) = \log_a(mn) and log⁡a(m)−log⁡a(n)=log⁡a(mn)\log_a(m) - \log_a(n) = \log_a\left(\dfrac{m}{n}\right):

log⁡x(8⋅6⋅4⋅27⋅5⋅3)\log_x\left(\dfrac{8 \cdot 6 \cdot 4 \cdot 2}{7 \cdot 5 \cdot 3}\right)


Numerator: 8×6×4×2=3848 \times 6 \times 4 \times 2 = 384

Denominator: 7×5×3=1057 \times 5 \times 3 = 105

log⁡x(384105)\log_x\left(\dfrac{384}{105}\right)

=log⁡x(12835)= \log_x\left(\dfrac{128}{35}\right)

Therefore, the answer is log⁡x(12835)\log_x\left(\dfrac{128}{35}\right).

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