The equation of a curve is in the form of a cubic polynomial. The curve has a maximum point at and passes through the point with coordinates . The tangent to the curve at is parallel to the line with equation . Find .
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Worked solution
6
1. Set up the general equation for a cubic polynomial and find its derivative.
Let the cubic polynomial be .
The derivative is:
2. Use the coordinates of the maximum point to form two equations.
Since the curve has a maximum point at , it passes through and its gradient there is :
3. Use the other given point and the tangent gradient to form two more equations.
The curve passes through :
The tangent at is parallel to , so its gradient is :
4. Solve the system of linear equations to find $a, b, c$ and $d$.
Subtracting (2) from (4) to eliminate :
Substituting (5) into (2):
Subtracting (1) from (3) to eliminate :
Substituting (5) and (6) into (7):
Now find and :
From (1):
5. State the final cubic polynomial.
Substitute the values of and back into the generic cubic polynomial:
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