0606 P21 - Nov 2018 - Q2 - 7 marks
8515
(a) Solve
\(3^{\left(\frac{x}{2}-1\right)}=10.\)
(b) Solve
\(2e^{1-2y}=3e^{3y+2}.\)
Solution
Answer: (a) \(x=6.19\). (b) \(y=-0.281\).
(a) Take logarithms to base \(3\):
\(\frac{x}{2}-1=\log_3 10.\)
Therefore
\(\frac{x}{2}=1+\log_3 10, \qquad x=2(1+\log_3 10).\)
Using \(\log_3 10=\dfrac{\log 10}{\log 3}\),
\(x=2\left(1+\frac{\log 10}{\log 3}\right)=6.19\quad\text{to 3 significant figures}.\)
(b) Start with
\(2e^{1-2y}=3e^{3y+2}.\)
Divide by \(3e^{1-2y}\):
\(e^{(3y+2)-(1-2y)}=\frac{2}{3}.\)
So
\(e^{5y+1}=\frac{2}{3}.\)
Taking natural logarithms gives
\(5y+1=\ln\frac23.\)
Hence
\(y=\frac{\ln(2/3)-1}{5}=-0.281\quad\text{to 3 significant figures}.\)