How do you multiply #(6+ 9i ) ( 6- 9i )#?

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Answer 1 · 2016-10-17T16:58:34

I recommend the F.O.I.L method. Please see the explanation.
Answer: 117

The F.O.I.L. Method works well for this type of multiplication

First terms:

#6 xx 6 = 36#

Running sum:

36

Outside terms:

#6 xx -9i = -54i#

Running sum:

#36 - 54i#

Inside terms:

#6 xx 9i = 54i#

Running sum:

#36 - 54i + 54i = 36#

Last terms:

#9i xx -9i = -81i^2

Remember that #i^2 = -1#

#-81i^2 = 81#

Running sum:

36 + 81 = 117