6+7=73.57+8=8411+12=12612+24=390.624+28=?
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The answer to the question
You have 9i in the denominator. The goal is to have a real denominator. 9i is imaginary. i * i = -1 which is real. If you multiply the denominator by i, the denominator will be real. Since this is a fraction, we multiply the numerator and denominator by i.