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  • Structured Proof

    Structured proof that \(\sqrt{2}\) is not a rational number.

    Rational numbers \(\in\) \(\mathbb{R}\) can be written as \(\frac{a}{b}\), where a and b are positive integers.
    The quotient of two rational numbers is a rational number.

    Outline of Proof by contradiction

    \(P = \sqrt{2}\) is not a rational number.
    Suppose \(\sim P\) which means not \(P\), we have \(\sim P\) = \(\sqrt{2}\) is a rational number.
    Is so \(\sqrt{2} = \frac{a}{b}\) and for any integer lets say \(I\), \(I - \sqrt{2} = \frac{c}{d}\) where a,b,c and d are positive integers and b,d cannot equal 0.
    If \(I = 5\) using integer \(5\),
    \[5-\sqrt{2}= \frac{c}{d},\]
    adding the square root of \(2\) shows \[5 = \frac{c}{d} + \sqrt{2}\]
    subtracting the fraction we have \[5-\frac{c}{d} = \sqrt{2}\]
    \[\frac{5}{1} - \frac{c}{d} = \sqrt{2}\]
    \[\sqrt{2} = \frac{5d-c}{d}\]
    There are no integers for c and d to make this equation true.
    Therefore \(\sim P\) is false and \(P = \sqrt{2}\) is not a rational number is true.

    Two functions \(f,g\) have contact order k at x_0

    (a) Construct a structured proof to show that \(f(x):=\cos(x)\) and \(g(x):=\sqrt{1-x^2}\) have contact order 4 at \(x_0=0\).




    We have \(f(x_0)=\cos(0)= 1\) and \(g(x_0):=\sqrt{1-0^2}=1\) So it obeys the first rule.

    \[\frac{d^i f(x)}{dx^i} |_{x=x_0} = \frac{d^i g(x)}{dx^i} |_{x=x_0},\]

    Lets say \(i = 1, x_0 = 0, f(x) = cos(x), g(x) = \sqrt{1-x^2}\)

    \[\frac{d' f(x)}{dx'} \vert_{x=x_0} = \frac{d' g(x)}{dx'} \vert_{x=x_0}, i=1.\]

    \[\frac{d' cos(x)}{dx'} \vert_{x=x_0} = \frac{d' \sqrt{1-x^2}}{dx'} \vert_{x=x_0}, i=1.\]

    \[- \sin(0) = -\frac{0} {\sqrt{1-0^2}}, \\ 0 = 0\]

    It obeys the second rule.


    For \(1 \geq i \geq k, x_0 = 0 \) we have \[\frac{d^{k+1}f(x)}{dx^{k+1}}|x=x_0 \ne \frac{d^{k+1}g(x)}{dx^{k+1}}|x=x_0\] Lets say \(k = 4, f(x) = \cos(x), g(x) = \sqrt{1-x^2}\) \[\frac{d^{5}\cos(x)}{dx^{5}}|x=x_0 \ne \frac{d^{5}\sqrt{1-x^2}}{dx^{5}}|x=x_0\]

    \[-sin(0) \ne - \frac{15(0)(4(0)^2+3)}{(1-(0)^{2})^{\frac{9}{2}}}|x=x_0\]

    (b) Construct a structured proof to show that \(f(x):=\sin(x)\) and \(g(x):=x\) have contact order \(3\) at \(x_0=0\).




    We have \(f(x_0)=\sin(0)= 0\) and \(g(x_0):=0=0\) So it obeys the first rule.

    \[\frac{d^i f(x)}{dx^i} |_{x=x_0} = \frac{d^i g(x)}{dx^i} |_{x=x_0}, .\]

    Lets say \(i = 1, x_0 = 0, f(x) = sin(x), g(x) =x\)

    \[\frac{d' f(x)}{dx'} \vert_{x=x_0} = \frac{d' g(x)}{dx'} \vert_{x=x_0}, i=1.\]

    \[\frac{d' sin(x)}{dx'} \vert_{x=x_0} = \frac{d' x}{dx'} \vert_{x=x_0}, i=1.\]

    \[\cos(0) = 1 \\ 1 = 1\]

    It obeys the second rule.


    For \(1 \geq i \geq k, x_0 = 0 \) we have \[\frac{d^{k+1}f(x)}{dx^{k+1}}|x=x_0 \ne \frac{d^{k+1}g(x)}{dx^{k+1}}|x=x_0\] Lets say \(k = 3, f(x) = \sin(x), g(x) =x\) \[\frac{d^{4}\sin(x)}{dx^{4}}|x=x_0 \ne \frac{d^{4}x}{dx^{4}}|x=x_0\]

    \[sin(0) \ne \frac{d^{4}0}{d(0)^{4}}|x=x_0\]

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