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1. Set up the general equation for a cubic curve and find its derivative.
Let the equation of the cubic curve be y=ax3+bx2+cx+d.
Differentiating with respect to x:
dxdy=3ax2+2bx+c
2. Use the stationary point $(1, -6)$ to form two equations.
Since the point (1,−6) lies on the curve, we substitute x=1,y=−6 into the curve equation:
a(1)3+b(1)2+c(1)+d=−6
a+b+c+d=−6— (1)
Since (1,−6) is a stationary point, dxdy=0 at x=1:
3a(1)2+2b(1)+c=0
3a+2b+c=0— (2)
3. Use the points $(2, 3)$ and $(-3, -22)$ to form two more equations.
Substituting (2,3) into the curve's equation:
a(2)3+b(2)2+c(2)+d=3
8a+4b+2c+d=3— (3)
Substituting (−3,−22) into the curve's equation:
a(−3)3+b(−3)2+c(−3)+d=−22
−27a+9b−3c+d=−22— (4)
4. Eliminate $d$ to form a system of equations in terms of $a$, $b$, and $c$.
From (1), make d the subject:
d=−6−a−b−c
Substitute d into (3):
8a+4b+2c+(−6−a−b−c)=3
7a+3b+c=9— (5)
Substitute d into (4):
−27a+9b−3c+(−6−a−b−c)=−22
−28a+8b−4c=−16
Divide by −4:
7a−2b+c=4— (6)
5. Solve the reduced system for the remaining constants.
Subtract (6) from (5) to eliminate a and c:
(7a+3b+c)−(7a−2b+c)=9−4
5b=5⟹b=1
Substitute b=1 into (2) and (5):
From (2): 3a+2(1)+c=0⟹c=−3a−2— (7)
From (5): 7a+3(1)+c=9⟹c=6−7a— (8)
Equating (7) and (8):
−3a−2=6−7a
4a=8⟹a=2
Substitute a=2 back into (7):
c=−3(2)−2=−8
Substitute a,b, and c into the expression for d:
d=−6−2−1−(−8)=−1
6. Write the final equation of the cubic curve.
With a=2, b=1, c=−8, and d=−1, the equation of the cubic curve is:
y=2x3+x2−8x−1
答案y=2x3+x2−8x−1