1. Set up the general equation for a cubic polynomial and find its derivative.
Let the cubic polynomial be f(x)=ax3+bx2+cx+d.
The derivative is:
f′(x)=3ax2+2bx+c
2. Use the coordinates of the maximum point to form two equations.
Since the curve has a maximum point at (−1,0), it passes through (−1,0) and its gradient there is 0:
f(−1)=0⟹a(−1)3+b(−1)2+c(−1)+d=0
−a+b−c+d=0— (1)
f′(−1)=0⟹3a(−1)2+2b(−1)+c=0
3a−2b+c=0— (2)
3. Use the other given point and the tangent gradient to form two more equations.
The curve passes through (4,25):
f(4)=25⟹a(4)3+b(4)2+c(4)+d=25
64a+16b+4c+d=25— (3)
The tangent at x=3 is parallel to y=24x+5, so its gradient is 24:
f′(3)=24⟹3a(3)2+2b(3)+c=24
27a+6b+c=24— (4)
4. Solve the system of linear equations to find $a, b, c$ and $d$.
Subtracting (2) from (4) to eliminate c:
(27a+6b+c)−(3a−2b+c)=24−0
24a+8b=24⟹3a+b=3⟹b=3−3a— (5)
Substituting (5) into (2):
3a−2(3−3a)+c=0⟹c=6−9a— (6)
Subtracting (1) from (3) to eliminate d:
(64a+16b+4c+d)−(−a+b−c+d)=25−0
65a+15b+5c=25⟹13a+3b+c=5— (7)
Substituting (5) and (6) into (7):
13a+3(3−3a)+(6−9a)=5
13a+9−9a+6−9a=5
−5a+15=5⟹−5a=−10⟹a=2
Now find b,c, and d:
b=3−3(2)=−3
c=6−9(2)=−12
From (1): −2+(−3)−(−12)+d=0⟹7+d=0⟹d=−7
5. State the final cubic polynomial.
Substitute the values of a,b,c, and d back into the generic cubic polynomial:
f(x)=2x3−3x2−12x−7
答案f(x)=2x3−3x2−12x−7