ANSWER:

No, it is not valid if you did not complete the fatiha or you were not in ruku with him at the same time.

QUESTION: If i was praying whit the imam from the beginning for asr salah and in the 4 rakah i didnt manage to recite surah al fatiha to the end because i made a mistake in surah fatiha so i went back and corrected the ayah where i made mistake then i continued my recitation but then imam went to do rukuh and i knew even if i recited fast i wouldnt catch up because imam will raise his head. is rakah valid?