The diagram shows part of the curve y=2x+5y = \sqrt{2x+5}. A is a point on the curve and the x-coordinate of A is 2. The normal to the curve at A meets the x-axis at B.

  1. (a)

    Find the equation of the normal to the curve at A.

  2. (b)

    Find the area of the shaded region bounded by the normal ABAB, the curve, the x-axis and the y-axis.

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Worked solution

13(a)

1. Find the $y$-coordinate of point A

When x=2x = 2,
y=2(2)+5=9=3y = \sqrt{2(2) + 5} = \sqrt{9} = 3
So, point A is (2,3)(2, 3).

2. Differentiate the equation of the curve to find the gradient function

y=(2x+5)12y = (2x + 5)^{\frac{1}{2}}
dydx=12(2x+5)−12⋅2\frac{dy}{dx} = \frac{1}{2}(2x + 5)^{-\frac{1}{2}} \cdot 2
dydx=12x+5\frac{dy}{dx} = \frac{1}{\sqrt{2x + 5}}

3. Find the gradient of the tangent at A

At x=2x = 2,
dydx=12(2)+5=13\frac{dy}{dx} = \frac{1}{\sqrt{2(2) + 5}} = \frac{1}{3}

4. Find the gradient of the normal and its equation

Gradient of normal =−113=−3= -\frac{1}{\frac{1}{3}} = -3
Equation of the normal at (2,3)(2, 3):
y−3=−3(x−2)y - 3 = -3(x - 2)
y−3=−3x+6y - 3 = -3x + 6
y=−3x+9y = -3x + 9

Answer

y=−3x+9y = -3x + 9

13(b)

1. Find the equation of the normal at A and the coordinates of B (from part a).

dydx=12(2x+5)−12(2)=12x+5\frac{dy}{dx} = \frac{1}{2}(2x+5)^{-\frac{1}{2}}(2) = \frac{1}{\sqrt{2x+5}}
At x=2x = 2, dydx=19=13\frac{dy}{dx} = \frac{1}{\sqrt{9}} = \frac{1}{3}.
Gradient of normal =−3= -3.
Equation of normal: y−3=−3(x−2)  ⟹  y=−3x+9y - 3 = -3(x - 2) \implies y = -3x + 9.
At BB, y=0  ⟹  −3x+9=0  ⟹  x=3y = 0 \implies -3x + 9 = 0 \implies x = 3. So BB is (3,0)(3, 0).

2. Set up the expression for the total shaded area.

The shaded area is the sum of the area under the curve from x=0x = 0 to x=2x = 2 and the area of the triangle formed by the normal from x=2x = 2 to x=3x = 3.
Total Area=∫022x+5 dx+Area of triangle\text{Total Area} = \int_{0}^{2} \sqrt{2x+5} \, dx + \text{Area of triangle}

3. Evaluate the definite integral for the area under the curve.

∫02(2x+5)12 dx=[(2x+5)3232×2]02\int_{0}^{2} (2x+5)^{\frac{1}{2}} \, dx = \left[ \frac{(2x+5)^{\frac{3}{2}}}{\frac{3}{2} \times 2} \right]_{0}^{2}
=[13(2x+5)32]02= \left[ \frac{1}{3}(2x+5)^{\frac{3}{2}} \right]_{0}^{2}
=13(9)32−13(5)32= \frac{1}{3}(9)^{\frac{3}{2}} - \frac{1}{3}(5)^{\frac{3}{2}}
=273−553=9−553= \frac{27}{3} - \frac{5\sqrt{5}}{3} = 9 - \frac{5\sqrt{5}}{3}

4. Calculate the area of the triangle and the total area.

Area of triangle=12×base×height=12×(3−2)×3=1.5\text{Area of triangle} = \frac{1}{2} \times \text{base} \times \text{height} = \frac{1}{2} \times (3 - 2) \times 3 = 1.5
Total Area=9−553+1.5=10.5−553≈6.77\text{Total Area} = 9 - \frac{5\sqrt{5}}{3} + 1.5 = 10.5 - \frac{5\sqrt{5}}{3} \approx 6.77

Answer

10.5−55310.5 - \frac{5\sqrt{5}}{3} or 6.776.77 (to 3 s.f.)

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