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I am struggling to see what is happening here particularly when ## \displaystyle \sum_{i=1}^{n-1}## transforms into ##\displaystyle \int_{x_1^e}^{x_{n}^e}##

##\displaystyle 0=\sum_{i=1}^{n-1} \left [ \int_{x_i^e}^{x_{i+1}^e} (a \frac{dw}{dx} \frac{du}{dx}+cwu-wf )dx- \left [ w(x) a \frac{du}{dx} \right ]_{x_i^e}^{x_{i+1}^e} \right ]##

##\displaystyle =\int_{x_1^e}^{x_{n}^e} (a \frac{dw}{dx} \frac{du}{dx}+cwu-wf )dx -w(x_1^e) (-a \frac{du}{dx})_{x_1^e}-w(x_2^e) (a \frac{du}{dx})_{x_2^e}-w(x_2^e) (-a \frac{du}{dx})_{x_2^e}-w(x_3^e) (a \frac{du}{dx})_{x_3^e} ......-w(x_{n-1}^e) (-a \frac{du}{dx})_{x_{n-1}^e}-w(x_{n}^e) (a \frac{du}{dx})_{x_{n}^e}##

Could anyone shed some light on what is happening..?

Apparently it is carried out in intervals of ##(x_1^e, x_2^e), (x_2^e, x_3^e),(x_{n-1}^e, x_n^e)##

Thanks