The Casio fx-991EX is an advanced scientific calculator that features a high-resolution natural textbook display, making it easy to read and input complex mathematical expressions. It offers over 552 functions, including those for calculus, statistics, and matrix operations. The calculator also supports 4-variable regression analysis and has a computer algebra system (CAS) for symbolic calculations. With both solar and battery power options, it?s reliable in various environments. Its user-friendly interface and robust functionality make it a popular choice for students and professionals in STEM fields.