The equation of a curve is .
- (i)
Find the coordinates of the stationary points of the curve.
- (ii)
Use the second derivative test to determine the nature of each of these points.
- (iii)
Find the range of values of for which is increasing.
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Worked solution
7(i)
1. Expand the equation of the curve and find the first derivative
2. Set the derivative to zero to find the $x$-coordinates of the stationary points
For stationary points, :
or
3. Find the corresponding $y$-coordinates
When ,
When ,
4. State the coordinates of the stationary points
The coordinates of the stationary points are and .
and
7(ii)
1. Find the second derivative of the curve
From part (i), .
2. Apply the second derivative test to the first stationary point
At :
Since , the point is a minimum point.
3. Apply the second derivative test to the second stationary point
At :
Since , the point is a maximum point.
is a minimum point and is a maximum point.
7(iii)
1. Set up the condition for the function to be increasing
For to be increasing, .
From part (i), .
2. Solve the quadratic inequality
The roots are and . Since the coefficient of is negative, the expression is positive between the roots.
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