0606 P22 - Nov 2018 - Q4 - 8 marks
8528
Solve
(i) \(2^{3x-1}=6,\)
(ii) \(\log_3(y+14)=1+\frac{2}{\log_y3}.\)
Solution
Answer: (i) \(x=1.19\). (ii) \(y=\dfrac73\).
(i) Take logarithms:
\((3x-1)\log2=\log6.\)
So
\(3x-1=\frac{\log6}{\log2}.\)
Hence
\(x=\frac{\frac{\log6}{\log2}+1}{3} =1.19\)
to 3 significant figures.
(ii) Use
\(1=\log_3 3 \quad\text{and}\quad \frac{1}{\log_y3}=\log_3y.\)
Then
\(\log_3(y+14)=\log_33+2\log_3y.\)
Therefore
\(\log_3(y+14)=\log_3(3y^2).\)
So
\(y+14=3y^2.\)
This gives
\(3y^2-y-14=0.\)
Factorise:
\((3y-7)(y+2)=0.\)
The logarithms require \(y\gt 0\), so \(y=-2\) is not valid. Hence
\(\boxed{y=\frac73}.\)