How do you multiply (1+3i)(1-3i) (1+3i)(13i) in trigonometric form?

1 Answer
Mar 27, 2016

(1 + 3i)(1 - 3i) = 10(1+3i)(13i)=10

Explanation:

abs(1+3i) = abs(1-3i) = sqrt(1^2+3^2) = sqrt(10)|1+3i|=|13i|=12+32=10

1 + 3i = sqrt(10) cis (arctan(3))1+3i=10cis(arctan(3))

1 - 3i = sqrt(10) cis (arctan(-3)) = sqrt(10) cis (-arctan(3))13i=10cis(arctan(3))=10cis(arctan(3))

So:

(1 + 3i)(1 - 3i)(1+3i)(13i)

= (sqrt(10) cis (arctan(3)))(sqrt(10) cis (-arctan(3)))=(10cis(arctan(3)))(10cis(arctan(3)))

= (sqrt(10))^2 cis (arctan(3) - arctan(3))=(10)2cis(arctan(3)arctan(3))

= 10 cis(0)=10cis(0)

= 10=10