A cubic curve passes through the points (2, 3) and (-3, -22). Find the equation of the curve if it has a stationary point at (1, -6).

Work through it with AI →

It won't just hand you the answer — it asks where you're stuck.How the checker works

Worked solution

1

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+dy = ax^3 + bx^2 + cx + d.
Differentiating with respect to xx:
dydx=3ax2+2bx+c\frac{dy}{dx} = 3ax^2 + 2bx + c

2. Form equations using the coordinates of the given points on the curve.

Substitute the coordinates of the points (1,−6)(1, -6), (2,3)(2, 3), and (−3,−22)(-3, -22) into the curve's equation:
At (1,−6)(1, -6):
a(1)3+b(1)2+c(1)+d=−6a(1)^3 + b(1)^2 + c(1) + d = -6
a+b+c+d=−6— (1)a + b + c + d = -6 \quad \text{--- (1)}

At (2,3)(2, 3):
a(2)3+b(2)2+c(2)+d=3a(2)^3 + b(2)^2 + c(2) + d = 3
8a+4b+2c+d=3— (2)8a + 4b + 2c + d = 3 \quad \text{--- (2)}

At (−3,−22)(-3, -22):
a(−3)3+b(−3)2+c(−3)+d=−22a(-3)^3 + b(-3)^2 + c(-3) + d = -22
−27a+9b−3c+d=−22— (3)-27a + 9b - 3c + d = -22 \quad \text{--- (3)}

3. Form an equation using the stationary point condition.

The curve has a stationary point at x=1x = 1, which means dydx=0\frac{dy}{dx} = 0 when x=1x = 1.
3a(1)2+2b(1)+c=03a(1)^2 + 2b(1) + c = 0
3a+2b+c=0— (4)3a + 2b + c = 0 \quad \text{--- (4)}

4. Solve the simultaneous equations to find $a$, $b$, $c$, and $d$.

Subtract (1) from (2) to eliminate dd:
(8a+4b+2c+d)−(a+b+c+d)=3−(−6)(8a + 4b + 2c + d) - (a + b + c + d) = 3 - (-6)
7a+3b+c=9— (5)7a + 3b + c = 9 \quad \text{--- (5)}

Subtract (1) from (3) to eliminate dd:
(−27a+9b−3c+d)−(a+b+c+d)=−22−(−6)(-27a + 9b - 3c + d) - (a + b + c + d) = -22 - (-6)
−28a+8b−4c=−16-28a + 8b - 4c = -16
Divide by −4-4:
7a−2b+c=4— (6)7a - 2b + c = 4 \quad \text{--- (6)}

Subtract (6) from (5) to eliminate aa and cc:
(7a+3b+c)−(7a−2b+c)=9−4(7a + 3b + c) - (7a - 2b + c) = 9 - 4
5b=5  ⟹  b=15b = 5 \implies b = 1

Substitute b=1b = 1 into (5) and (4):
7a+3(1)+c=9  ⟹  7a+c=6— (7)7a + 3(1) + c = 9 \implies 7a + c = 6 \quad \text{--- (7)}
3a+2(1)+c=0  ⟹  3a+c=−2— (8)3a + 2(1) + c = 0 \implies 3a + c = -2 \quad \text{--- (8)}

Subtract (8) from (7):
4a=8  ⟹  a=24a = 8 \implies a = 2

Substitute a=2a = 2 into (8):
3(2)+c=−2  ⟹  c=−83(2) + c = -2 \implies c = -8

Substitute a=2,b=1,c=−8a = 2, b = 1, c = -8 into (1):
2+1−8+d=−6  ⟹  d=−12 + 1 - 8 + d = -6 \implies d = -1

5. State the final equation of the curve.

Substituting the values of a,b,ca, b, c, and dd back into the general equation:
y=2x3+x2−8x−1y = 2x^3 + x^2 - 8x - 1

Answer

y=2x3+x2−8x−1y = 2x^3 + x^2 - 8x - 1

AI analysis — may contain mistakes.

Worked this one out on paper? Snap your steps and Gigle's AI math checker will circle the step where it went wrong.

More questions on this topic

  1. 1The curve C has equation x+y=(x−y)2x + y = (x - y)^2. It is given that C has only one turning point. Show that 1−dydx=21+2x−2y1 - \frac{dy}{dx} = \frac{2}{1 + 2x - 2y}…
  2. 2The curve C has equation x+y=(x−y)2.x + y = (x - y)^2. It is given that C has only one turning point. Show that 1−dydx=21+2x−2y1 - \dfrac{dy}{dx} = \dfrac{2}{1 + 2x - 2y}…
  3. 3A curve C1C_1 has the equation p2x2−y2=p2p^2x^2 - y^2 = p^2 where p>1p > 1. Sketch C1C_1, stating the coordinates of any points o…
  4. 4It is given that f(x)=ax3+bx2+cx+df(x)=ax^3+bx^2+cx+d, where aa, bb, cc and dd are constants. The curve CC with equation y=f(x)y=f(x)…
  5. 5The diagram shows the graphs of y=f(∣x∣)y = \mathrm{f}(|x|) and y=f′(x)y = \mathrm{f}'(x). The graph of y=f(∣x∣)y = \mathrm{f}(|x|) has t…