In pre-calculus and even in calculus, you may have been told that calculating the arccos of a number greater than 1 can’t be done. Since the cos function oscillates between -1 and 1, calculating arccos(3) would be “difficult.” When I ask my calculator to calculate arccos(3), it says “Error 0.” Of course the calculator and the early math courses are restricting themselves to the real number system.4 The arccos(3), for example, is just a complex number. In fact, it is pure imaginary, as we will now show.