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Worked solution

Use implicit tangents and a normal to bound an area

The curve has equation y2−xy=−1y^2-xy=-1.

(i) Find all tangent equations parallel to the y-axis.

(ii) State and justify whether the curve has any stationary points.

(iii) Find the area bounded by the axes and the normal at the point where y=2y=2.

[2013/YJC/I/7]

CJC · Discussion · Question 9

Answer

Vertical tangents x=±2x=\pm2; no stationary points; normal y=−3x/4+31/8y=-3x/4+31/8, bounded area 961/96961/96 square units.

(i) Find the vertical tangents

Implicit differentiation gives 2yy′−(y+xy′)=02yy^{\prime}-(y+xy^{\prime})=0, hence (2y−x)y′=y(2y-x)y^{\prime}=y. A vertical tangent requires 2y−x=02y-x=0. Substituting y=x/2y=x/2 in the curve gives −x2/4=−1-x^2/4=-1, so x=±2x=\pm2. At (2,1)(2,1) and (−2,−1)(-2,-1) the numerator yy is nonzero, so the tangent lines are x=2x=2 and x=−2x=-2.

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(ii) Rule out stationary points

A zero derivative would require y=0y=0, but this makes y2−xy=0y^2-xy=0, contradicting −1-1. The curve therefore has no stationary points.

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(iii) Find the normal and its intercept triangle

When y=2y=2, 4−2x=−14-2x=-1 gives x=5/2x=5/2. The tangent gradient is 2/(4−5/2)=4/32/(4-5/2)=4/3, so the normal gradient is −3/4-3/4. Its equation is

y−2=−34(x−52),y=−34x+318.y-2=-\frac34\left(x-\frac52\right),\qquad y=-\frac34x+\frac{31}{8}.

The intercepts are (0,31/8)(0,31/8) and (31/6,0)(31/6,0). The bounded region is a right triangle of area 12(31/6)(31/8)=961/96\tfrac12(31/6)(31/8)=961/96 square units.

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Common mistakes

The triangle is bounded by the normal and the axes, not by the curve.

Read the hint