If it has a function and its derivative nested together, reach for substitution first—like recognizing the old accounting forms in triplicate. But for products of unrelated functions, integration by parts is your only path, xu. In my coaching, they say to decide by looking for a clear chain, but life isn’t so neat. My granddaughter’s math app flashes problems in thirty seconds, and I still fumble; sometimes you must choose by the quiet weight of practice, not a rule.
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