Why Does the Sine Function Vanish in This Trigonometric Integration?

[physiker99]

I couldn't figure out how (*) derived. How can the underlined part's sine function vanishes?

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[Quinzio]

How much is the sine of pi, 2pi, 3pi, 4pi... n pi ?

 

[physiker99]

Hey Quinzio, thanks for the reply, but I have figured it out.
sin (n*pi) vanishes since n is always integer..