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Help With Imaginary Numbers?

Im sorry if this is long, but I need help. Thank you so much in advance! In my Algebra class we're learning about imaginary numbers, I have a test tomorrow, and I'm not sure if I have the hang of it. One of my assigned problems is - Simplify each expression: (2 + 4i) + (4 - i) and I got 6 + 3i. is that right? One is: 6 - (8 + 3i) and I don't know how to do that. Would it be -2 + 3i? One is (-2i)(5i) and I don't know how to multiply them. One is (9+4i)2 ( as in squared). Any help is greatly appreciated.

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1) (2 + 4i) + (4 - i) =
6 + 3i. ANSWER
2) 6 - (8 + 3i) =
6 - 8 - 3i =
-2 - 3i. ANSWER
3) (-2i)(5i) =
(-2)(5)(i)(i) =
-10(-1) =
10 ANSWER
4) (9 + 4i)^2 =
(9 + 4i)(9 + 4i) = Use FOIL
36 + 9(4i) + 9(4i) + (4i)(4i) =
36 + 18i + 16(-1) =
20 + 18i. ANSWER

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1st one is correct. Second one would be -2-3i (negative needs to be carried through).3rd would be -10i^2. i^2 =-1 so it would be 10. 4th one needs to be multiplied out using the foil method then use the i^2=-1 and plug in. Also, i=sqrt (-1).

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