Multiplying Complex Numbers: Z3 = 5 -j2 times Z4 = 3 + j

myristate

New member
Joined
Jan 19, 2016
Messages
3
So im trying to work out a mock question. I have attempted it below but honestly im not confident and i cant find good source material that covers negatives. Looking for some checking and correction

Given Values:

Z3 = 5 -j2
Z4 = 3 + j

Question:

Calculate Z3*Z4


My answer:

(5 -j2) (3 + j)

5(3 + j) + j2(3 + j)

(15 + j5) + (j6 + j²2)

15 + j11 + (-1) * 2

15 + j11 + 2

17 + j11
 
Everything looks fine, except one small sign error. You started with the number Z3 = 5 - 2j, so your very first step when you say 5(3 + j) + j2(3 + j) is wrong. Correct that, and everything else will fall into place and give you the right answer.
 
Coefficients are written to the immediate left of the variables/variable expressions.

- - - ---- -- - - -- -


Given Values:

Z3 = 5 - 2j
Z4 = 3 + j

Question:

Calculate Z3*Z4


New solution:

(5 - 2j)(3 + j) =

5(3 + j) - 2j(3 + j) =

15 + 5j - 6j - 2j² =

15 - j - 2(-1) =

15 - j + 2 =

17 - j
 
So im trying to work out a mock question. I have attempted it below but honestly im not confident and i cant find good source material that covers negatives. Looking for some checking and correction

Given Values:

Z3 = 5 -j2
Z4 = 3 + j

Question:

Calculate Z3*Z4


My answer:

(5 -j2) (3 + j)

5(3 + j) + j2(3 + j) Not correct

(15 + j5) + (j6 + j²2)

15 + j11 + (-1) * 2

15 + j11 + 2

17 + j11
Sorry but (-1) * 2 is NOT +2
 
Top