Khan/khan-exercises — pull request #58391 by mauk81 (2013-08-02 11:07:58+00:00)
Title: ensure that one of the numbers has a imaginary component

example: [adding_and_subtracting_complex_numbers.html?seed=120](http://sandcastle.khanacademy.org/media/castles/Khan:master/exercises/adding_and_subtracting_complex_numbers.html?seed=120&problem=0&debug)