The diagram below shows part of the graph of y=x2+2x+5x+3y = \frac{x^2 + 2x + 5}{x + 3}. Find the coordinates of the minimum point PP.

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Worked solution

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1. Find the derivative $\frac{dy}{dx}$ using the quotient rule.

Let u=x2+2x+5u = x^2 + 2x + 5 and v=x+3v = x + 3.
Then dudx=2x+2\frac{du}{dx} = 2x + 2 and dvdx=1\frac{dv}{dx} = 1.

dydx=vdudx−udvdxv2\frac{dy}{dx} = \frac{v\frac{du}{dx} - u\frac{dv}{dx}}{v^2}
dydx=(x+3)(2x+2)−(x2+2x+5)(1)(x+3)2\frac{dy}{dx} = \frac{(x + 3)(2x + 2) - (x^2 + 2x + 5)(1)}{(x + 3)^2}
dydx=2x2+2x+6x+6−x2−2x−5(x+3)2\frac{dy}{dx} = \frac{2x^2 + 2x + 6x + 6 - x^2 - 2x - 5}{(x + 3)^2}
dydx=x2+6x+1(x+3)2\frac{dy}{dx} = \frac{x^2 + 6x + 1}{(x + 3)^2}

2. Set the derivative to zero to find the $x$-coordinates of the stationary points.

For a minimum point, dydx=0\frac{dy}{dx} = 0.
x2+6x+1(x+3)2=0\frac{x^2 + 6x + 1}{(x + 3)^2} = 0
x2+6x+1=0x^2 + 6x + 1 = 0

3. Solve the quadratic equation for $x$.

Using the quadratic formula:
x=−6±62−4(1)(1)2(1)x = \frac{-6 \pm \sqrt{6^2 - 4(1)(1)}}{2(1)}
x=−6±322x = \frac{-6 \pm \sqrt{32}}{2}
x=−6±422x = \frac{-6 \pm 4\sqrt{2}}{2}
x=−3±22x = -3 \pm 2\sqrt{2}

4. Determine the correct $x$-coordinate for point $P$ based on the graph.

The graph has a vertical asymptote at x=−3x = -3. The point PP lies on the branch of the curve that crosses the positive yy-axis, which means x>−3x > -3.
Therefore, we reject x=−3−22x = -3 - 2\sqrt{2} and accept x=−3+22x = -3 + 2\sqrt{2}.

5. Find the corresponding $y$-coordinate for point $P$.

Substitute x=−3+22x = -3 + 2\sqrt{2} into the original equation:
y=(−3+22)2+2(−3+22)+5(−3+22)+3y = \frac{(-3 + 2\sqrt{2})^2 + 2(-3 + 2\sqrt{2}) + 5}{(-3 + 2\sqrt{2}) + 3}
y=9−122+8−6+42+522y = \frac{9 - 12\sqrt{2} + 8 - 6 + 4\sqrt{2} + 5}{2\sqrt{2}}
y=16−8222y = \frac{16 - 8\sqrt{2}}{2\sqrt{2}}
y=8−422y = \frac{8 - 4\sqrt{2}}{\sqrt{2}}
y=82−82=42−4y = \frac{8\sqrt{2} - 8}{2} = 4\sqrt{2} - 4

The coordinates of PP are (−3+22,−4+42)(-3 + 2\sqrt{2}, -4 + 4\sqrt{2}).

Answer

(−3+22,−4+42)(-3 + 2\sqrt{2}, -4 + 4\sqrt{2})

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