Date | November 2017 | Marks available | 8 | Reference code | 17N.1.hl.TZ0.7 |
Level | HL only | Paper | 1 | Time zone | TZ0 |
Command term | Determine | Question number | 7 | Adapted from | N/A |
Question
The folium of Descartes is a curve defined by the equation x3+y3−3xy=0, shown in the following diagram.
Determine the exact coordinates of the point P on the curve where the tangent line is parallel to the y-axis.
Markscheme
x3+y3−3xy=0
3x2+3y2dydx−3xdydx−3y=0 M1A1
Note: Differentiation wrt y is also acceptable.
dydx=3y−3x23y2−3x (=y−x2y2−x) (A1)
Note: All following marks may be awarded if the denominator is correct, but the numerator incorrect.
y2−x=0 M1
EITHER
x=y2
y6+y3−3y3=0 M1A1
y6−2y3=0
y3(y3−2)=0
(y≠0)∴y=3√2 A1
x=(3√2)2 (=3√4) A1
OR
x3+xy−3xy=0 M1
x(x2−2y)=0
x≠0⇒y=x22 A1
y2=x44
x=x44
x(x3−4)=0
(x≠0)∴x=3√4 A1
y=(3√4)22=3√2 A1
[8 marks]