GNU Emacs Calc 2.02 Manual
3.7.7 Algebraic Entry Tutorial Exercise 3
The result from 1 RET 0 / will be the formula 1 / 0.
The "function" `/' cannot be evaluated when its second argument
is zero, so it is left in symbolic form. When you now type 0 *,
the result will be zero because Calc uses the general rule that "zero
times anything is zero."
The m i command enables an infinite mode in which 1 / 0
results in a special symbol that represents "infinity." If you
multiply infinity by zero, Calc uses another special new symbol to
show that the answer is "indeterminate." See section 5.5 Infinities, for
further discussion of infinite and indeterminate values.