While both sec ( pi/2) and tan ( pi/2) are undefined, they can be represented as 1/0. However, if sec ( pi/2) is equal to tan ( pi/2), then the following equation should be true: sec² ( pi/2) - tan² ( pi/2) = 0. But this equation simplifies to 1 = 0, which is obviously false. This leads to two possible explanations: (i) sec ( pi/2) is not equal to tan ( pi/2), or (ii) the formula sec² ( phi) - tan² ( phi) cannot be used for phi = k pi + pi/2 (where k is an integer).