0606 P23 - Nov 2021 - Q4 - 8 marks
8066
(a) Solve the equation
\(\log_6(2x-3)=\frac12.\)
Give your answer in exact form.
(b) Solve the equation
\(\ln 2u-\ln(u-4)=1.\)
Give your answer in exact form.
(c) Solve the equation
\(\frac{3^v}{27^{2v-5}}=9.\)
Solution
Answer: \(x=\frac{3+\sqrt6}{2}\), \(u=\frac{4e}{e-2}\), \(v=\frac{13}{5}\).
(a) From
\(\log_6(2x-3)=\frac12,\)
write the equation in exponential form:
\(2x-3=6^{1/2}.\)
So
\(2x-3=\sqrt6.\)
Therefore
\(x=\frac{3+\sqrt6}{2}.\)
(b) Use the logarithm law \(\ln A-\ln B=\ln\frac AB\):
\(\ln\left(\frac{2u}{u-4}\right)=1.\)
Since \(1=\ln e\),
\(\frac{2u}{u-4}=e.\)
Hence
\(2u=e(u-4).\)
So
\(2u=eu-4e.\)
Rearranging,
\(u(2-e)=-4e.\)
Therefore
\(u=\frac{4e}{e-2}.\)
(c) Since \(27=3^3\) and \(9=3^2\),
\(\frac{3^v}{27^{2v-5}}=9\)
becomes
\(\frac{3^v}{(3^3)^{2v-5}}=3^2.\)
So
\(\frac{3^v}{3^{6v-15}}=3^2.\)
Using index laws,
\(3^{v-(6v-15)}=3^2.\)
Thus
\(3^{15-5v}=3^2.\)
Equating powers,
\(15-5v=2.\)
Therefore
\(v=\frac{13}{5}.\)