MHB Is this integral substitution approach correct for evaluating the integral I?

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The integral I is evaluated using the substitution \( u = y - 2 \), transforming the limits from 2 to 6 into 0 to 4. The integral simplifies to \( I = \int_{0}^{4} \frac{u+2}{\sqrt{u}} \, du \), which is further broken down into two parts: \( \int_{0}^{4} u^{1/2} \, du \) and \( 2 \int_{0}^{4} u^{-1/2} \, du \). After performing the calculations, the result confirms that \( I = \frac{40}{3} \). The substitution approach and subsequent calculations are deemed correct. The method effectively demonstrates the evaluation of the integral.
karush
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$\large{S6.7.r.44}$
$$\displaystyle
I=\int_{2}^{6}\frac{y}{\sqrt{y-2}} \,dy = \frac{40}{3}$$
$$
\begin{align}
u&=y-2 &y&=u+2 \\
du&=dy
\end{align}$$
then
$$\displaystyle
I=\int_{0}^{4}\frac{u+2}{\sqrt{u}} \, du
=\int_{0}^{4}{u}^{1/2} \, du + 2\int_{0}^{4} {u}^{-1/2} \, du$$
Just seeing if going in right direction...
 
Last edited:
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Looks good.
 
$\large{S6.7.r.44}$
$$\displaystyle
I=\int_{2}^{6}\frac{y}{\sqrt{y-2}} \,dy = \frac{40}{3}$$
$$
\begin{align}
u&=y-2 &y&=u+2 \\
du&=dy
\end{align}$$
then
$$\displaystyle
I=\int_{0}^{4}\frac{u+2}{\sqrt{u}} \, du
=\int_{0}^{4}{u}^{1/2} \, du
+ 2\int_{0}^{4} {u}^{-1/2} \, du
=\frac{ \sqrt{2}u(u+4)}{4 } \\
\text{back subst and calc gives} \\
I=\frac{40}{3}$$
 
Last edited:
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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