Considering Minato can be a dunce at times. I couldn't imagine him learning FTG on his own...
Hiruzen knew all the jutsu of the village so its only logical to say that Prime Hiruzen taught Minato FTG and Rasengan
Hiruzen knew all the jutsu of the village so its only logical to say that Prime Hiruzen taught Minato FTG and Rasengan