0606 P12 - Mar 2020 - Q11 - 7 marks
8083
Given that
\(\int_1^a\left(\frac2{2x+3}+\frac3{3x-1}-\frac1x\right)\,dx=\ln2.4\)
and that \(a\gt 1\), find the value of \(a\).
Solution
Answer: \(a=3\).
Integrate term by term:
\(\int\left(\frac2{2x+3}+\frac3{3x-1}-\frac1x\right)\,dx =\ln(2x+3)+\ln(3x-1)-\ln x.\)
So
\(\left[\ln(2x+3)+\ln(3x-1)-\ln x\right]_1^a=\ln2.4.\)
Substitute the limits:
\(\ln(2a+3)+\ln(3a-1)-\ln a-(\ln5+\ln2)=\ln2.4.\)
Using logarithm laws,
\(\ln\left(\frac{(2a+3)(3a-1)}{10a}\right)=\ln2.4.\)
Therefore
\(\frac{(2a+3)(3a-1)}{10a}=2.4.\)
Since \(2.4=\frac{12}{5}\),
\((2a+3)(3a-1)=24a.\)
Expand:
\(6a^2+7a-3=24a.\)
So
\(6a^2-17a-3=0.\)
Use the quadratic formula:
\(a=\frac{17\pm\sqrt{(-17)^2-4(6)(-3)}}{12} =\frac{17\pm\sqrt{361}}{12}.\)
Thus
\(a=\frac{17\pm19}{12}.\)
So
\(a=3\quad\text{or}\quad a=-\frac16.\)
Given that \(a\gt 1\),
\(a=3.\)