0606 P23 - Nov 2022 - Q4 - 6 marks
7910
The line \(y=kx+6\) intersects the curve \(y=x^3-4x^2+3kx+2\) at the point where \(x=2\).
(a) Find the value of \(k\).
(b) Show that the line intersects the curve at only one point.
Solution
Answer: \(k=3\), and the remaining quadratic factor has no real roots.
Answer: \(k=3\), and the line intersects the curve at only one point.
At an intersection,
\(kx+6=x^3-4x^2+3kx+2.\)
Substitute \(x=2\):
\(2k+6=8-16+6k+2.\)
So
\(2k+6=6k-6,\)
which gives
\(k=3.\)
Now substitute \(k=3\) into the intersection equation:
\(3x+6=x^3-4x^2+9x+2.\)
Hence
\(x^3-4x^2+6x-4=0.\)
Since \(x=2\) is a root, factorise:
\(x^3-4x^2+6x-4=(x-2)(x^2-2x+2).\)
The quadratic factor has discriminant
\((-2)^2-4(1)(2)=4-8=-4\lt 0.\)
So it has no real roots. Therefore the only real intersection occurs at \(x=2\), and the line intersects the curve at only one point.