Complex sentence: His teachers recognized his talents in mathematics, but by 15 years of age he had already learned all the material taught at the school, and he began to study differential calculus from the works of Euler and Lagrange.
Split into simpler sentences: His teachers recognized his talents in mathematics, but by 15 years of age he had already learned all the material taught at the school. He then began to study differential calculus from the works of Euler and Lagrange.