Ask a question

How do you simplify this answer, keeping in mind that we are working with imaginary numbers? (8i)(4i)(-9i)

Do you use a different version of FOIL?
Do you just multiply 8x4x(-9)?
or do you multiply 8iX4i which equals 32 but then mutliply it by -1 because i^2=-1 so you'd have -32, and then multiply that by -9i which would equal -288i. Is that the final answer?

2 Answers by Expert Tutors

Tutors, sign in to answer this question.
Britt E. | Wash U Law Grad for Law, Test Prep, Math, Spanish and English TutoringWash U Law Grad for Law, Test Prep, Math...
4.9 4.9 (33 lesson ratings) (33)
Actually Ashley forgot to cube the i terms, not just multiply the coefficients. If you multiply this out you get -288i3
i3= -i
so -288(-i) = 288i
i = √-1
so the answer is 288i or 288√-1
Ashley J. | Mathematics Tutor!Mathematics Tutor!
When multiplying (8i)(4i)(9i), you can multiply (8i)(4i), which is equal to -32. You then multiply (-32)(9i), which equals -288i. However, if you chose to multiply (4i)(9i) = -36, and then multiply (-36)(8i) you also get -288i.
-288i is the final answer.


Hi Ashley;
Fellow tutor here...Do I understand correctly?  If you multiply an i number by another i number, the result is a non-i number?