0606 P21 - Jun 2021 - Q7 - 6 marks
7978
Find the exact values of the constant \(k\) for which the line
\(y=2x+1\)
is a tangent to the curve
\(y=4x^2+kx+k-2.\)
Solution
Answer: \(k=10\pm\sqrt{48}\).
The key point is to differentiate first, then substitute the required value or solve the resulting equation.
At an intersection of the line and the curve,
\(4x^2+kx+k-2=2x+1.\)
Rearrange:
\(4x^2+(k-2)x+k-3=0.\)
For the line to be a tangent, this quadratic must have equal roots. Therefore its discriminant is zero:
\((k-2)^2-4(4)(k-3)=0.\)
Expand and simplify:
\(k^2-4k+4-16k+48=0.\)
So
\(k^2-20k+52=0.\)
Using the quadratic formula,
\(k=\frac{20\pm\sqrt{(-20)^2-4(1)(52)}}{2}.\)
Thus
\(k=\frac{20\pm\sqrt{192}}{2} =10\pm\sqrt{48}.\)