The Geometry of the Cubic
Imagine you are standing on the graph of the function f(x)=2x3−9ax2+12a2x+1. Because the leading coefficient is positive, this is a dynamic, sweeping path that climbs from negative infinity, reaches a peak, descends into a valley, and then surges upward again.
The points where this curve turns—the peak and the valley—are our local maximum and local minimum. We are told the maximum occurs at p and the minimum at q, with the constraint that p2=q.
Let us embark on the journey to find f(3).
The Calculus of Motion
To find these turning points, we need to identify where the slope of the tangent line is perfectly horizontal. We calculate the first derivative, f′(x):
f′(x)=dxd(2x3−9ax2+12a2x+1)
Applying the power rule, we obtain:
This derivative acts as our compass. To find the critical points, we set f′(x)=0:
Dividing by 6 simplifies our task immensely:
The Algebra of Extrema
Next, we factorize this quadratic equation. We look for two numbers that multiply to 2a2 and add to −3a, which are −a and −2a. Thus, the equation becomes:
This yields two critical points: x=a and x=2a. Since we are given that a>0, we know for certain that a<2a.
For a cubic with a positive leading coefficient, the first critical point from the left is the local maximum, and the second is the local minimum. Therefore, p=a and q=2a.
The Constraint and the Final Calculation
We are armed with the condition p2=q. Substituting our values, we get:
Rearranging this, we have a2−2a=0, which factors to a(a−2)=0. Since the problem explicitly states a>0, we must reject a=0.
Thus, a=2. With a determined, our function is fully revealed:
f(x)=2x3−9(2)x2+12(22)x+1
Finally, we evaluate f(3):
f(3)=2(3)3−18(3)2+48(3)+1
The journey from the abstract geometry of a cubic curve to the concrete value of 37 is complete.