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The curve with equation $y = x^2 - 32 ext{√}(x) + 20$, $x > 0$ has a stationary point $P$ - Edexcel - A-Level Maths Pure - Question 4 - 2013 - Paper 4

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The-curve-with-equation----$y-=-x^2---32-ext{√}(x)-+-20$,----$x->-0$----has-a-stationary-point-$P$-Edexcel-A-Level Maths Pure-Question 4-2013-Paper 4.png

The curve with equation $y = x^2 - 32 ext{√}(x) + 20$, $x > 0$ has a stationary point $P$. Use calculus (a) to find the coordinates of $P$, (b) to ... show full transcript

Worked Solution & Example Answer:The curve with equation $y = x^2 - 32 ext{√}(x) + 20$, $x > 0$ has a stationary point $P$ - Edexcel - A-Level Maths Pure - Question 4 - 2013 - Paper 4

Step 1

(a) to find the coordinates of P

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Answer

To find the coordinates of the stationary point PP, we first need to calculate the first derivative of the equation.

  1. Find the first derivative:
    dydx=2x3212x=2x16x\frac{dy}{dx} = 2x - 32\cdot\frac{1}{2\sqrt{x}} = 2x - \frac{16}{\sqrt{x}}

  2. Set the first derivative to zero:
    2x16x=02x - \frac{16}{\sqrt{x}} = 0

  3. Solve for xx:
    Rearranging gives us:
    2x=16x2x = \frac{16}{\sqrt{x}}

    Squaring both sides leads to:
    4x2=164x^2 = 16

    Thus:
    x2=4x=2x^2 = 4 \rightarrow x = 2

  4. Substitute xx back to find yy:
    y=22322+20y = 2^2 - 32\cdot\sqrt{2} + 20

    Calculating gives:
    y=4322+20=24322y = 4 - 32\cdot\sqrt{2} + 20 = 24 - 32\cdot\sqrt{2}

So, the coordinates of the stationary point PP are (2,24322)(2, 24 - 32\cdot\sqrt{2}).

Step 2

(b) to determine the nature of the stationary point P

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Answer

To determine the nature of the stationary point PP, we will analyze the second derivative.

  1. Find the second derivative:
    d2ydx2=2+162x3/2=2+8x3/2\frac{d^2y}{dx^2} = 2 + \frac{16}{2x^{3/2}} = 2 + \frac{8}{x^{3/2}}

  2. Evaluate the second derivative at x=2x = 2:
    d2ydx2x=2=2+8(2)3/2=2+822=2+22\frac{d^2y}{dx^2}\Big|_{x=2} = 2 + \frac{8}{(2)^{3/2}} = 2 + \frac{8}{2\sqrt{2}} = 2 + 2\sqrt{2}

    Since both terms are positive, we find that
    d2ydx2>0\frac{d^2y}{dx^2} > 0

This indicates that the stationary point PP is a minimum.

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