The gradient function of a curve y=f(x)y = f(x) is given by 36(2x+1)2\frac{36}{(2x+1)^2} for x>−12x > -\frac{1}{2}. The curve passes through the point (12,2)\left(\frac{1}{2}, 2\right).

  1. (a)

    Find the equation of the curve.

  2. (b)

    Find the angle, in degrees, that the tangent to the curve at x=1x = 1 makes with the xx-axis.

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Worked solution

6(a)

1. Integrate the gradient function to find the general equation of the curve

y=∫36(2x+1)2 dxy = \int \frac{36}{(2x+1)^2} \, dx
y=∫36(2x+1)−2 dxy = \int 36(2x+1)^{-2} \, dx
y=36(2x+1)−1(−1)(2)+cy = \frac{36(2x+1)^{-1}}{(-1)(2)} + c
y=−182x+1+cy = -\frac{18}{2x+1} + c

2. Substitute the given point to find the constant of integration

Substitute x=12x = \frac{1}{2} and y=2y = 2:
2=−182(12)+1+c2 = -\frac{18}{2(\frac{1}{2})+1} + c
2=−182+c2 = -\frac{18}{2} + c
2=−9+c2 = -9 + c
c=11c = 11

3. State the final equation of the curve

y=−182x+1+11y = -\frac{18}{2x+1} + 11

Answer

y=−182x+1+11y = -\frac{18}{2x+1} + 11

6(b)

1. Find the gradient of the tangent at $x = 1$.

At x=1x = 1,
dydx=36(2(1)+1)2\frac{dy}{dx} = \frac{36}{(2(1)+1)^2}
dydx=3632=369=4\frac{dy}{dx} = \frac{36}{3^2} = \frac{36}{9} = 4

2. Calculate the angle the tangent makes with the $x$-axis.

Let θ\theta be the angle the tangent makes with the xx-axis.
tan⁡θ=4\tan \theta = 4
θ=tan⁡−1(4)\theta = \tan^{-1}(4)
θ≈75.964∘\theta \approx 75.964^\circ
θ=76.0∘ (to 1 d.p.)\theta = 76.0^\circ \text{ (to 1 d.p.)}

Answer

76.0∘76.0^\circ

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