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tad1 Group Title

The derivative of an even function is an odd function. the derivative of an odd function is an even function. Prove these results from the limit definition of the derivative: lim(as x approaches zero) [f(x) - f(a)]/(x -a)

  • 3 years ago
  • 3 years ago

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  1. extibiano Group Title
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    Well it takes some math to prove that by that definition... However if you look at the rule f(x)=x^n then f'(x)=nx^(n-1) it's very clear. If you have the even function x^4 it's derivative is 4(x^3) that is an odd function. It's derivative is 12(x^2), an even function.

    • 3 years ago
  2. tad1 Group Title
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    Unfortunately, the problem was to use the definition. but I think I've got it. thanks for trying/

    • 3 years ago
  3. Tangent Group Title
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    Another way you could prove this easily is by using trigonometric terms and identities, i.e. if you solved this, \[\lim_{x \rightarrow 0} [\sin(x+h)-\sin (x)]/h\] you would get cos x out as the answer. The sine function is an odd function, cosine an even, so it would be proven by that.

    • 3 years ago
  4. ac7qz Group Title
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    It comes from the definition of even and odd. If a function is even, f(x) = f(-x). (If it's odd, f(x) = -f(-x).) You can use the product rule, since f(-x) = f(-1 . x), so f' = (-1)' . x + x' . (-1) = -x' Since f'(x) = -f'(-x), then the derivative is an odd function. A similar argument can be used for odd functions.

    • 3 years ago
  5. hburgiel Group Title
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    I think ac7qz has the right idea, but it looks like there might be an error in the calculation.

    • 3 years ago
  6. katarina Group Title
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    Let f(x) be an odd function (f(x)=-f(-x)) $$f'(x_0)=\lim_{x \rightarrow x_0} \dfrac{f(x)-f(x_0)}{x-x_0}=\lim_{-x \rightarrow -x_0} \dfrac{-f(-x)+f(-x_0)}{-x_0-(-x)}=\lim_{y \rightarrow- x_0} \dfrac{-f(y)+f(-x_0)}{-x_0-y}= $$ $$ =\lim_{y \rightarrow- x_0} \dfrac{f(y)-f(-x_0)}{y-(-x_0)}=f'(-x_0) $$ So we proved that $$ f′(x_0)=f′(−x_0)$$ That means that the derivative of an odd function is even. Let f(x) be an even function (f(x)=f(-x)) $$f'(x_0)=\lim_{x \rightarrow x_0} \dfrac{f(x)-f(x_0)}{x-x_0}=\lim_{-x \rightarrow -x_0} \dfrac{f(-x)-f(-x_0)}{-x_0-(-x)}=\lim_{y \rightarrow- x_0} \dfrac{f(y)-f(-x_0)}{-x_0-y}= $$ $$ =-\lim_{y \rightarrow- x_0} \dfrac{f(y)-f(-x_0)}{y-(-x_0)}=-f'(-x_0) $$ So we proved that $$ f′(x_0)=-f′(−x_0)$$ That means that the derivative of an even function is odd. The only assumption we need is the existense of the derivatives of the regarded function in some area around f(x0) and f(-x0).

    • 3 years ago
  7. emceepaul Group Title
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    Can we not do this with the definition \[f'(-x)= \lim_{h \to 0}\frac{f(-x+h) - f(x)} {h} = \lim_{h \to 0}\frac{f(-(x-h)) - f(x)} {h} = \lim_{h \to 0}\frac{f((x-h)) - f(x)} {h}=\\ \lim_{-h \to 0}\frac{f((x-(-h)) - f(x)} {-h}= -\lim_{h \to 0}\frac{f((x+h)) - f(x)} {h}= \\-f'(x)\]

    • one year ago
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