Worked solution
Use implicit tangents and a normal to bound an area
The curve has equation .
(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 .
[2013/YJC/I/7]
CJC · Discussion · Question 9
Answer
Vertical tangents ; no stationary points; normal , bounded area square units.
(i) Find the vertical tangents
Implicit differentiation gives , hence . A vertical tangent requires . Substituting in the curve gives , so . At and the numerator is nonzero, so the tangent lines are and .
(ii) Rule out stationary points
A zero derivative would require , but this makes , contradicting . The curve therefore has no stationary points.
(iii) Find the normal and its intercept triangle
When , gives . The tangent gradient is , so the normal gradient is . Its equation is
The intercepts are and . The bounded region is a right triangle of area square units.
Common mistakes
The triangle is bounded by the normal and the axes, not by the curve.