qqp question1: What makes the fundamental theorem of calculus so fundamental? question2: Why is there a variable (usually x) in the Second Fundamental Theorem of Calculus? Wouldn't it be just as true if the theorem were defined as true for any interval 'a' to 'b', instead of 'a' to 'x'? What does the x signify, is it possible that the SFT could be defined another way?