Welcome to the TeX.SE. community. Do you have, please, the .pdf source of your picture? However there is in math-mode the font CM= computer modern. The text is similar to Garamond.
– SebastianoJan 21 '21 at 16:54
According to Acrobat Reader, this book uses some Computer Modern Maths fonts , and also Courier, Garamond (roman, bold, italic), Helvetica, Latin Modern sans & Latin Modern mono.
– BernardJan 21 '21 at 17:46
pdffontson it. – Davislor Jan 21 '21 at 17:08