Question

1. For i = −1, if 3i(2 + 5i) = x + 6i, then x = ?
A. –15 B. 5 C. 5i D. 15i E. 27i
please explain. :D

Answer 1

What do you get when you distribute 3i(2 + 5i)

Answer 2

5i+15i?

Answer 3

6i* i ment

Answer 4

6i is correct, but it should be 15i^2 since i*i = i^2

Answer 5

so it's really

6i + 15i^2

Answer 6

but what is i^2 really?

Answer 7

1?

Answer 8

close

Answer 9

\[\Large i = \sqrt{-1}\]

square both sides

Answer 10

i do not know.

Answer 11

squaring a square root effectively undoes it

Answer 12

and they cancel

Answer 13

\[\Large i = \sqrt{-1}\]

\[\Large i^2 = \left(\sqrt{-1}\right)^2\]

\[\Large i^2 = -1\]

Answer 14

then?

Answer 15

so 15i^2 is really 15(-1) = -15

Answer 16

ohh

Answer 17

3i(2 + 5i)

6i + 15i^2

6i + 15(-1)

6i + (-15)

-15 + 6i

Answer 18

3i(2 + 5i) = x + 6i

-15 + 6i = x + 6i

I'm sure you can see what x is

Answer 19

Thank you.

Answer 20

yw