For a particular curve d2ydx2=3x−2\frac{\text{d}^2 y}{\text{d}x^2} = 3x - 2. The tangent to the curve at the point A(4,−40)A(4, -40) is parallel to the line 3x−y=23x - y = 2. Find the equation of the curve.

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Worked solution

4

1. Integrate the second derivative to find the first derivative

dydx=∫(3x−2) dx\frac{\text{d}y}{\text{d}x} = \int (3x - 2) \, \text{d}x
dydx=32x2−2x+c\frac{\text{d}y}{\text{d}x} = \frac{3}{2}x^2 - 2x + c

2. Determine the gradient of the tangent at $x = 4$

The line is 3x−y=2  ⟹  y=3x−23x - y = 2 \implies y = 3x - 2.
The gradient of this line is 33.
Since the tangent is parallel to this line, dydx=3\frac{\text{d}y}{\text{d}x} = 3 at x=4x = 4.

3. Find the constant of integration $c$

Substitute x=4x = 4 and dydx=3\frac{\text{d}y}{\text{d}x} = 3:
3=32(4)2−2(4)+c3 = \frac{3}{2}(4)^2 - 2(4) + c
3=24−8+c3 = 24 - 8 + c
3=16+c  ⟹  c=−133 = 16 + c \implies c = -13
So, dydx=32x2−2x−13\frac{\text{d}y}{\text{d}x} = \frac{3}{2}x^2 - 2x - 13

4. Integrate the first derivative to find the equation of the curve

y=∫(32x2−2x−13) dxy = \int \left(\frac{3}{2}x^2 - 2x - 13\right) \, \text{d}x
y=12x3−x2−13x+dy = \frac{1}{2}x^3 - x^2 - 13x + d

5. Find the constant of integration $d$ using the point $A(4, -40)$

Substitute x=4x = 4 and y=−40y = -40:
−40=12(4)3−(4)2−13(4)+d-40 = \frac{1}{2}(4)^3 - (4)^2 - 13(4) + d
−40=32−16−52+d-40 = 32 - 16 - 52 + d
−40=−36+d  ⟹  d=−4-40 = -36 + d \implies d = -4

6. State the final equation of the curve

y=12x3−x2−13x−4y = \frac{1}{2}x^3 - x^2 - 13x - 4

Answer

y=12x3−x2−13x−4y = \frac{1}{2}x^3 - x^2 - 13x - 4

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