MHB Indefinite integral with two parts

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The integral \(\int e^{4\ln{x}}x^2 dx\) presents challenges for u-substitution and integration by parts. It is noted that \(e^{4\ln{x}}\) simplifies to \(x^4\), which changes the approach to the integration. The discussion emphasizes the importance of simplifying expressions before applying integration techniques. Participants suggest that recognizing algebraic identities can facilitate the integration process. Overall, simplifying the integral leads to a more straightforward solution.
find_the_fun
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I'm trying to integrate [math]\int e^{4\ln{x}}x^2 dx[/math]
I can't see using u-substition, [math]x^2[/math] isn't the derivative of [math]e^{4\ln{x}}[/math] nor vice-versa.

I tried integrating by parts and that didn't work. I used [math]u=e^{4\ln{x}}[/math] and [math]dv=x^2 dx[/math]

I know I can't rewrite [math]e^{4\ln{x}}[/math] as [math]e^4e^\ln{x}[/math]
 
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Re: indefinite integral with two parts

find_the_fun said:
I'm trying to integrate [math]\int e^{4\ln{x}}x^2 dx[/math]
I can't see using u-substition, [math]x^2[/math] isn't the derivative of [math]e^{4\ln{x}}[/math] nor vice-versa.

I tried integrating by parts and that didn't work. I used [math]u=e^{4\ln{x}}[/math] and [math]dv=x^2 dx[/math]

Note that $e^{4\ln x} = e^{\ln(x^4)} = x^4$.

Can you take things from here?
 
Re: indefinite integral with two parts

Chris L T521 said:
Note that $e^{4\ln x} = e^{\ln(x^4)} = x^4$.

Can you take things from here?

I guess the lesson learned from this is to simplify the expression algebraically before attempting integration techniques.
 
There are probably loads of proofs of this online, but I do not want to cheat. Here is my attempt: Convexity says that $$f(\lambda a + (1-\lambda)b) \leq \lambda f(a) + (1-\lambda) f(b)$$ $$f(b + \lambda(a-b)) \leq f(b) + \lambda (f(a) - f(b))$$ We know from the intermediate value theorem that there exists a ##c \in (b,a)## such that $$\frac{f(a) - f(b)}{a-b} = f'(c).$$ Hence $$f(b + \lambda(a-b)) \leq f(b) + \lambda (a - b) f'(c))$$ $$\frac{f(b + \lambda(a-b)) - f(b)}{\lambda(a-b)}...

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