Finding $f$ When $6\int_{1}^{x} f(t)\, dt+5=3x \, f(x)-x^3$

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Discussion Overview

The discussion revolves around finding the function \( f \) defined on the interval \([1, \infty)\) that satisfies the equation \( 6\int_{1}^{x} f(t)\, dt + 5 = 3x \, f(x) - x^3 \) for all \( x \geq 1 \). Participants explore the implications of this equation, including specific evaluations and transformations related to the function.

Discussion Character

  • Mathematical reasoning
  • Technical explanation
  • Homework-related

Main Points Raised

  • Post 1 introduces the equation and asks for the value of \( f(2) \) and the Laplace transform \( \mathcal{L} \{ f(t)\} \).
  • Post 2 questions the validity of the equation by noting that substituting \( x=1 \) leads to a contradiction, suggesting a possible error in the formulation.
  • Post 3 acknowledges a typo in the equation and corrects it, reaffirming the original equation presented in Post 1.
  • Post 4 derives a first-order ordinary differential equation (ODE) from the corrected equation and states the initial condition \( f(1)=0 \), although this contradicts the initial condition given in Post 1.

Areas of Agreement / Disagreement

There is no consensus on the correctness of the initial conditions or the derived ODE, as participants have conflicting interpretations of the problem and its requirements.

Contextual Notes

The discussion reveals a potential inconsistency regarding the initial condition \( f(1)=2 \) versus \( f(1)=0 \) as derived in Post 4. The implications of these conditions on the solution remain unresolved.

sbhatnagar
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Let $f:[1,\infty)\to [2,\infty)$ be a differentiable function such that $f(1)=2$. If

$$ 6\int_{1}^{x} f(t)\, dt+5=3x \, f(x)-x^3$$

for all $x \geq 1$, then:

1) Find the value of $f(2)$.

2) Find $\mathcal{L} \{ f(t)\}$.
 
Last edited:
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sbhatnagar said:
Let $f:[1,\infty)\to [2,\infty)$ be a differentiable function such that $f(1)=2$. If

$$ 6\int_{1}^{x} f(t)\, dt=3x \, f(x)-x^3$$

for all $x \geq 1$, then:

1) Find the value of $f(2)$.

2) Find $\mathcal{L} \{ f(t)\}$.
Something wrong here? If $x=1$, then the equation $\displaystyle 6\int_{1}^{x} f(t)\, dt=3x \, f(x)-x^3$ becomes $0=5.$
 
Sorry, I made a typo. The equation is

$$6\int_{1}^{x}f(t)dt+5=3xf(x)-x^3$$

I am really sorry about this.(Sadface)
 
sbhatnagar said:
... the equation is...

$$6\int_{1}^{x}f(t)dt+5=3xf(x)-x^3$$

...

Deriving both terms You arrive to the linear first term ODE...

$\displaystyle \frac{d}{d x}\ f(x)= \frac{f(x)}{x} + x$ (1)

… with initial condition' $f(1)=0$ and the solving procedure is 'standard'...

Kind regards

$\chi$ $\sigma$
 
Last edited:

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