The diagram shows part of the graph of y=22+xy = 2\sqrt{2 + x}. It cuts the x-axis at (−2,0)(-2,0). The line y=2xy = 2x meets the curve at PP. The normal to the curve at PP meets the x-axis at RR.

  1. (a)

    Find the coordinates of RR.

  2. (b)

    Show that OPR is an isosceles triangle.

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Worked solution

14(a)

1. Find the coordinates of $P$ by equating the line and the curve

2x=22+x2x = 2\sqrt{2 + x}
x=2+xx = \sqrt{2 + x}
x2=2+xx^2 = 2 + x
x2−x−2=0x^2 - x - 2 = 0
(x−2)(x+1)=0(x - 2)(x + 1) = 0
Since x≥0x \ge 0 for x=2+xx = \sqrt{2+x}, x=2x = 2.
When x=2x = 2, y=2(2)=4y = 2(2) = 4.
So, PP is (2,4)(2, 4).

2. Find the gradient function of the curve

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

3. Calculate the gradient of the tangent and normal at $P$

At x=2x = 2, gradient of tangent mt=12+2=12m_t = \frac{1}{\sqrt{2 + 2}} = \frac{1}{2}.
Gradient of normal mn=−1mt=−2m_n = -\frac{1}{m_t} = -2.

4. Find the equation of the normal at $P$

y−4=−2(x−2)y - 4 = -2(x - 2)
y=−2x+4+4y = -2x + 4 + 4
y=−2x+8y = -2x + 8

5. Find the coordinates of $R$ by setting $y = 0$

At RR, the normal meets the x-axis, so y=0y = 0.
0=−2x+80 = -2x + 8
2x=82x = 8
x=4x = 4
The coordinates of RR are (4,0)(4, 0).

Answer

(4,0)(4, 0)

14(b)

1. Find the coordinates of $P$ by solving the equations of the curve and the line simultaneously.

2x=22+x2x = 2\sqrt{2 + x}
x=2+xx = \sqrt{2 + x}
x2=2+xx^2 = 2 + x
x2−x−2=0x^2 - x - 2 = 0
(x−2)(x+1)=0(x - 2)(x + 1) = 0
Since PP is in the first quadrant, x=2x = 2.
y=2(2)=4y = 2(2) = 4
So, PP is (2,4)(2, 4).

2. Find the coordinates of $R$ by finding the equation of the normal at $P$.

y=2(2+x)12y = 2(2 + x)^{\frac{1}{2}}
dydx=2(12)(2+x)−12=12+x\frac{dy}{dx} = 2 \left(\frac{1}{2}\right)(2 + x)^{-\frac{1}{2}} = \frac{1}{\sqrt{2 + x}}
At x=2x = 2, gradient of tangent mt=14=12m_t = \frac{1}{\sqrt{4}} = \frac{1}{2}.
Gradient of normal mn=−1mt=−2m_n = -\frac{1}{m_t} = -2.
Equation of normal at P(2,4)P(2, 4):
y−4=−2(x−2)y - 4 = -2(x - 2)
y=−2x+8y = -2x + 8
At RR, y=0y = 0:
0=−2x+8  ⟹  x=40 = -2x + 8 \implies x = 4.
So, RR is (4,0)(4, 0).

3. Calculate the lengths of the sides of triangle $OPR$ to show it is isosceles.

Coordinates are O(0,0)O(0, 0), P(2,4)P(2, 4), and R(4,0)R(4, 0).
Length of OP=(2−0)2+(4−0)2=4+16=20OP = \sqrt{(2 - 0)^2 + (4 - 0)^2} = \sqrt{4 + 16} = \sqrt{20}
Length of PR=(4−2)2+(0−4)2=22+(−4)2=4+16=20PR = \sqrt{(4 - 2)^2 + (0 - 4)^2} = \sqrt{2^2 + (-4)^2} = \sqrt{4 + 16} = \sqrt{20}
Length of OR=4OR = 4
Since OP=PR=20OP = PR = \sqrt{20}, triangle OPROPR has two equal sides and is therefore an isosceles triangle.

Answer

Since OP=PR=20OP = PR = \sqrt{20}, triangle OPROPR is an isosceles triangle.

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