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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+y33xy=0, shown in the following diagram.

N17/5/MATHL/HP1/ENG/TZ0/07

Determine the exact coordinates of the point P on the curve where the tangent line is parallel to the y-axis.

Markscheme

x3+y33xy=0

3x2+3y2dydx3xdydx3y=0     M1A1

 

Note:     Differentiation wrt y is also acceptable.

 

dydx=3y3x23y23x (=yx2y2x)     (A1)

 

Note:     All following marks may be awarded if the denominator is correct, but the numerator incorrect.

 

y2x=0     M1

EITHER

x=y2

y6+y33y3=0     M1A1

y62y3=0

y3(y32)=0

(y0)     A1

x = {\left( {\sqrt[3]{2}} \right)^2}{\text{ }}\left( { = \sqrt[3]{4}} \right)     A1

OR

{x^3} + xy - 3xy = 0     M1

x({x^2} - 2y) = 0

x \ne 0 \Rightarrow y = \frac{{{x^2}}}{2}     A1

{y^2} = \frac{{{x^4}}}{4}

x = \frac{{{x^4}}}{4}

x({x^3} - 4) = 0

(x \ne 0)\therefore x = \sqrt[3]{4}     A1

y = \frac{{{{\left( {\sqrt[3]{4}} \right)}^2}}}{2} = \sqrt[3]{2}     A1

[8 marks]

Examiners report

[N/A]

Syllabus sections

Topic 6 - Core: Calculus » 6.2 » Implicit differentiation.

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