0606 P11 - Jun 2020 - Q5 - 6 marks
8101
Find the equation of the tangent to the curve
\(y=\frac{\ln(3x^2-1)}{x+2}\)
at the point where \(x=1\). Give your answer in the form \(y=mx+c\), where \(m\) and \(c\) are constants correct to 3 decimal places.
Solution
Answer: \(y=0.923x-0.692\).
Let
\(y=\frac{\ln(3x^2-1)}{x+2}.\)
Using the quotient rule,
\(\frac{dy}{dx} =\frac{(x+2)\left(\frac{6x}{3x^2-1}\right)-\ln(3x^2-1)}{(x+2)^2}.\)
When \(x=1\),
\(y=\frac{\ln(3(1)^2-1)}{1+2} =\frac{\ln2}{3}.\)
The gradient at \(x=1\) is
\(\frac{dy}{dx} =\frac{3\left(\frac{6}{2}\right)-\ln2}{9} =\frac{9-\ln2}{9}.\)
So
\(m=1-\frac{\ln2}{9}=0.92298\ldots.\)
Hence \(m=0.923\) to 3 decimal places.
The tangent has equation
\(y-\frac{\ln2}{3}=m(x-1).\)
Thus
\(c=\frac{\ln2}{3}-m.\)
Using \(m=\frac{9-\ln2}{9}\),
\(c=\frac{\ln2}{3}-\frac{9-\ln2}{9} =\frac{4\ln2}{9}-1.\)
So
\(c=-0.6919\ldots,\)
and hence \(c=-0.692\) to 3 decimal places.
Therefore the tangent is
\(y=0.923x-0.692.\)