1. Set up the general equation for the 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. Form equations using the coordinates of the given points on the curve.
Substitute the coordinates of the points (1,−6), (2,3), and (−3,−22) into the curve's equation:
At (1,−6):
a(1)3+b(1)2+c(1)+d=−6
a+b+c+d=−6— (1)
At (2,3):
a(2)3+b(2)2+c(2)+d=3
8a+4b+2c+d=3— (2)
At (−3,−22):
a(−3)3+b(−3)2+c(−3)+d=−22
−27a+9b−3c+d=−22— (3)
3. Form an equation using the stationary point condition.
The curve has a stationary point at x=1, which means dxdy=0 when x=1.
3a(1)2+2b(1)+c=0
3a+2b+c=0— (4)
4. Solve the simultaneous equations to find $a$, $b$, $c$, and $d$.
Subtract (1) from (2) to eliminate d:
(8a+4b+2c+d)−(a+b+c+d)=3−(−6)
7a+3b+c=9— (5)
Subtract (1) from (3) to eliminate d:
(−27a+9b−3c+d)−(a+b+c+d)=−22−(−6)
−28a+8b−4c=−16
Divide by −4:
7a−2b+c=4— (6)
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 (5) and (4):
7a+3(1)+c=9⟹7a+c=6— (7)
3a+2(1)+c=0⟹3a+c=−2— (8)
Subtract (8) from (7):
4a=8⟹a=2
Substitute a=2 into (8):
3(2)+c=−2⟹c=−8
Substitute a=2,b=1,c=−8 into (1):
2+1−8+d=−6⟹d=−1
5. State the final equation of the curve.
Substituting the values of a,b,c, and d back into the general equation:
y=2x3+x2−8x−1
答案y=2x3+x2−8x−1