Final answer:
Both Cosimo de'Medici and Filippo Brunelleschi were associated with Florence's artistic community.
Step-by-step explanation:
Both Cosimo de'Medici and Filippo Brunelleschi received their early artistic training in Florence. Cosimo de'Medici was a patron rather than a direct student, and it was Donatello who was deeply sponsored by Cosimo de'Medici. Filippo Brunelleschi, although not under the direct tutelage of Donatello, did collaborate and study Roman art and architecture on a trip to Rome with him, which greatly influenced his own work.
However, to address who instructed them, Lorenzo Ghiberti is known for leading a workshop where many famous artists and sculptors trained, including Donatello and later on, Michelozzo and Paolo Uccello. This workshop was renowned for the revival of ancient techniques, and both Brunelleschi and Donatello would have been exposed to these methods and teachings through their experiences in Florence's vibrant artistic community.
The answer to the question 'Cosimo de'Medici and Filippo Brunelleschi both studied in Florence under the instruction of who?' is D. Lorenzo Ghiberti.