SOLUTION: (5i)(−3i) I just do not understand.

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Question 1017026: (5i)(−3i)
I just do not understand.
Found 2 solutions by macston, ikleyn:
Answer by macston(5194) About Me  (Show Source):
You can put this solution on YOUR website!
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+i=sqrt%28-1%29
You have to accept this. It is a definition, a given.
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So i%5E2=-1
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%285i%29%28-3i%29=5%2A-3%2Ai%2Ai=-15i%5E2=%28-15%29%28-1%29=15

Answer by ikleyn(52786) About Me  (Show Source):
You can put this solution on YOUR website!
.
See an introductory lesson on complex numbers and operations on them
Complex numbers and arithmetic operations on them in this site.

At the end of that lesson there is a list of other my lessons on complex numbers in this site.