A curve is such that d2ydx2=4−3x\dfrac{d^2y}{dx^2} = 4 - 3x and the point P(2,5)P(2,5) lies on the curve. The gradient of the curve at PP is 2. Find the equation of the curve.

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Worked solution

6

1. Integrate the second derivative to find the general expression for the first derivative.

dydx=∫(4−3x) dx\frac{dy}{dx} = \int (4 - 3x) \, dx
dydx=4x−32x2+c\frac{dy}{dx} = 4x - \frac{3}{2}x^2 + c

2. Use the given gradient at point P to find the constant of integration $c$.

At P(2,5)P(2, 5), dydx=2\frac{dy}{dx} = 2.
2=4(2)−32(2)2+c2 = 4(2) - \frac{3}{2}(2)^2 + c
2=8−6+c2 = 8 - 6 + c
c=0c = 0
So, dydx=4x−32x2\frac{dy}{dx} = 4x - \frac{3}{2}x^2

3. Integrate the first derivative to find the general equation of the curve.

y=∫(4x−32x2) dxy = \int \left(4x - \frac{3}{2}x^2\right) \, dx
y=2x2−12x3+dy = 2x^2 - \frac{1}{2}x^3 + d

4. Use the coordinates of point P to find the second constant of integration $d$.

At P(2,5)P(2, 5), y=5y = 5.
5=2(2)2−12(2)3+d5 = 2(2)^2 - \frac{1}{2}(2)^3 + d
5=8−4+d5 = 8 - 4 + d
d=1d = 1
So, y=2x2−12x3+1y = 2x^2 - \frac{1}{2}x^3 + 1

Answer

y=2x2−12x3−2x+5y = 2x^2 - \frac{1}{2}x^3 - 2x + 5

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