Limits

Calculus Level 4

lim x 0 f ( x ) f ( x ) + sin x x 3 + e x \lim_{x\to0} \dfrac{ f'(x) }{ f'''(x) + \frac{\sin x}{\sqrt[3]{x}} + e^x}

Let f f be a differentiable function on R \mathbb R with continuous derivatives til order 4. Suppose f ( x ) = f ( x ) x R f(-x) = f(x) \forall x \in \mathbb R . Compute the limit above.


The answer is 0.

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1 solution

Kishore S. Shenoy
Mar 17, 2016

Given f ( x ) = f ( x ) f(x) = f(-x) That means, f ( x ) f(x) is an even function. f ( x ) = a 2 n x 2 n + a 2 n 2 x 2 n 2 + + a 4 x 4 + a 2 x 2 f ( x ) = + 2 a 2 x f ( x ) = + 24 a 4 x f ( 0 ) = 0 f ( 0 ) = 0 f(x) = a_{2n}x^{2n} +a_{2n-2}x^{2n-2}+\cdots + a_4x^4 + a_2x^2\\f'(x) = \cdots + 2a_2x\\f'''(x)=\cdots + 24a_4x\\\Rightarrow f'(0) = 0\\f'''(0) = 0

Also, lim x 0 sin x x 3 = lim x 0 x 2 3 sin x x = 0 × 1 = 0 \lim_{x\to0}\dfrac{\sin x}{\sqrt[3] x} = \lim_{x\to0}\dfrac{\sqrt[3] {x^2}\sin x}{ x} = 0\times 1 = 0 lim x 0 e x = 1 \lim_{x\to0}e^x = 1

Therefore, lim x 0 f ( x ) f ( x ) + sin x x 3 + e x = 0 0 + 0 + 1 = 0 \lim_{x\to0} \dfrac{ f'(x) }{ f'''(x) + \frac{\sin x}{\sqrt[3]{x}} + e^x} = \dfrac0{0+0+1} = 0

Moderator note:

Good approach.

After showing that the numerator is zero, we still have to show that the denominator is non-zero, in order to conclude that the limit is zero.

Did the same

Sudhamsh Suraj - 4 years, 3 months ago

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High Five friend!

Kishore S. Shenoy - 4 years, 3 months ago

@Kishore S Shenoy The question said f is an even function : it needn't be a polynomial at all.

Venkata Karthik Bandaru - 4 years, 3 months ago

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Ya... That's true... Thanks. I need to think...

Kishore S. Shenoy - 4 years, 3 months ago

Hey, but all even functions can be represented like this using McLaren Series... So it should be right... Point me out if I'm wrong...

Kishore S. Shenoy - 4 years, 3 months ago

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http://math.stackexchange.com/questions/553080/can-any-continuous-function-be-represented-as-an-infinite-polynomial

Polynomials of finite degree are different from formal power series. Furthermore, MacLaren is far from representing all possible even functions.

Venkata Karthik Bandaru - 4 years, 3 months ago

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