Which Formula Should I Use for Calculating Power in an AC Circuit?

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hquang001
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TL;DR
When the teacher ask me to find real power of an AC Circuit, which formula should i use ?
Should i use P = V.I or P = Vrms.Irms.cosφ ?
 
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Discussion

Real or average or active power = Vrms.Irms.cosφ (W)
Instantaneous power = V.I (VA)
 

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