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i know the law (fg)'=f'g+fg'
but here there is epsilon
and it turns to someother sign i dont know what it says.
i cant understand the transition
?
There's no epsilon in what you posted. Are you talking about [itex]\prod[/itex]? Note that this is upper-case pi, which is different from the constant [itex]\pi[/itex], lower-case pi.
i know the law (fg)'=f'g+fg'
but here there is epsilon
and it turns to someother sign i dont know what it says.
i cant understand the transition
?