A curve has the equation .
- ((i))
Find the coordinates of the stationary point of the curve.
- ((ii))
Determine the nature of the stationary point.
- ((iii))
Hence sketch the graph of , showing all the intercepts.
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Worked solution
7((i))
1. Find the derivative of the curve equation
2. Set the derivative to zero to find the $x$-coordinate of the stationary point
For stationary points,
3. Find the corresponding $y$-coordinate
When ,
The coordinates of the stationary point are .
7((ii))
1. Recall the first derivative from part (i)
From part (i), .
The stationary point is at .
2. Apply the first derivative test around x = 4
Let's test values of slightly less than and slightly greater than 4.
When , .
When , .
3. Conclude the nature of the stationary point
Since the gradient is negative on both sides of the stationary point, the sign of the gradient does not change.
Therefore, the point is a stationary point of inflexion.
Stationary point of inflexion
7((iii))
1. Find the $y$-intercept
When ,
The -intercept is .
2. Find the $x$-intercept
When ,
The -intercept is .
3. Sketch the graph
Draw a cubic curve with a negative leading coefficient (decreasing overall). The curve passes through the -axis at , flattens out at the stationary point of inflexion , and crosses the -axis at approximately .
A cubic curve passing through and , with a stationary point of inflexion at .
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