0606 P21 - Nov 2024 - Q5 - 5 marks
7243
Given that \(\log _{a}(p+1)+\frac{1}{\log _{p} a}-\log _{a}(p+2)+\log _{a} 5=\log _{a} 12\), find the value of \(p\).
Solution
Answer: \(p=3\).
Apply the laws of logarithms carefully, paying attention to the base of each logarithm and any restrictions on the argument.
Use \(\frac{1}{\log_pa}=\log_ap\). The equation becomes
\(\log_a(p+1)+\log_ap-\log_a(p+2)+\log_a5=\log_a12.\)
Combining logarithms,
\(\log_a\left(\frac{5p(p+1)}{p+2}\right)=\log_a12.\)
Thus
\(\frac{5p(p+1)}{p+2}=12.\)
So \(5p^2+5p=12p+24\), giving
\(5p^2-7p-24=0.\)
Factorising,
\((5p+8)(p-3)=0.\)
The value \(p=-\frac85\) is not valid for the logarithms in the original equation, so \(p=3\).
This gives the required answer: \(p=3\).