Hi everyone. First of all, congratulations for the forum.

I'm trying to grasp, as a complete beginner, some concepts of Math. I'm reading "Mathematics: its content methods and meaning"

I have a doubt (it won't be the last (Happy))

Mathematics: Its Content, Methods and Meaning - Aleksandr Danilovich Aleksandrov, Andréi Nikoláevich Kolmogórov, M. A. Lavrent'ev - Google Libros

I hope the link works, if it doesn't I'll copy the example.

I don't understand the step in page 67

S_{2}=g(t + At)^{2}/ 2 = gt^{2}/2 + g/2(2tAt + AtIt's this part in bold that I don't know where it comes from.^{2})

I hope the notation is legible and my English readable.