Apostol’s Calculus was first published in the early 1960s (Vol 1 in 1961, Vol 2 in 1962). The "old version" typically refers to these first editions or the widely used 2nd editions from 1967/1969.
Apostol’s problems are rarely trivial. A student can spend days stuck on a single proof. The solucionario provides the "nudge" needed to see the trick or the algebraic manipulation required to proceed. For the old versions, where the pedagogy was arguably even stiffer, having the answer key is often the difference between quitting the book and finishing it.
Book details * ISBN-10. 0471000051. * ISBN-13. 978-0471000051. * Edition. 2nd. * Publisher. Wiley. * Publication date. January 16, Amazon.com Tom Apostol - Calculus vol.1 - The Swiss Bay