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Which is the product of the complex number (4+20i) and (4-5i)

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2 Answers

use the distributive property
(4 + 20i)(4-5i)= 4(4-5i) + 20i(4-5i)
get rid of parenthesis
16 -20i + 80i -100i2
i2=-1 substitute and group like terms then simplify
16 + -100(-1) + i(-20 + 80)
116 + 60i
 
(4+20i) x (4-5i)
factor out the 4 out
4( 1+5i) (4-5i)
multiply all the parts together
[4]x[1x4 + 1x(-5i) +(5i)x4 + (5i)(-5i)]
You multiply complex numbers similar to the way toy multiply binomials.
=[4]x[ 4 - 5i + 20i -25]
= 4(29+15i)