Date | November 2018 | Marks available | 4 | Reference code | 18N.1.SL.TZ0.S_10 |
Level | Standard Level | Paper | Paper 1 (without calculator) | Time zone | Time zone 0 |
Command term | Find and Hence | Question number | S_10 | Adapted from | N/A |
Question
Let f(x)=x3−2x2+ax+6. Part of the graph of f is shown in the following diagram.
The graph of f crosses the y-axis at the point P. The line L is tangent to the graph of f at P.
Find f′(x).
Hence, find the equation of L in terms of a.
The graph of f has a local minimum at the point Q. The line L passes through Q.
Find the value of a.
Markscheme
f′=3x2−4x+a A2 N2
[2 marks]
valid approach (M1)
eg f′(0)
correct working (A1)
eg 3(0)2−4(0)+a, slope = a, f′(0)=a
attempt to substitute gradient and coordinates into linear equation (M1)
eg y−6=a(x−0), y−0=a(x−6), 6=a(0)+c, L =ax+6
correct equation A1 N3
eg y=ax+6, y−6=ax, y−6=a(x−0)
[4 marks]
valid approach to find intersection (M1)
eg f(x)=L
correct equation (A1)
eg x3−2x2+ax+6=ax+6
correct working (A1)
eg x3−2x2=0, x2(x−2)=0
x=2 at Q (A1)
valid approach to find minimum (M1)
eg f′(x)=0
correct equation (A1)
eg 3x2−4x+a=0
substitution of their value of x at Q into their f′(x)=0 equation (M1)
eg 3(2)2−4(2)+a=0, 12−8+a=0
a = −4 A1 N0
[8 marks]