# Which of the following is not a complex number: sum of complex numbers, difference of complex numbers, product of complex numbers

hala718 | High School Teacher | (Level 1) Educator Emeritus

Posted on

let z and y are two complect numbers such that:

z= a + bi

y= c+ di

Now let us verify:

sum of compelx numbers:

(a+ bi) + (c+di) = (a+c) + (b+d)i

Then , the sum is a complex number.

Now the difference:

(a+ bi) - (c+di) = (a-c) + (b-d)i

The difference is a complex number.

Now the product:

(a+bi)*(c+di) = (ac + bci + adi + bi*di)

= ac + (bc+ad)i - bd

The product is a complex number

neela | High School Teacher | (Level 3) Valedictorian

Posted on

None of them need be complex number

Sum of two compex compex numbers: 2+3i and 2-3i has a sum 4 which is not acomplex number.

The difference of two complex numbers need not be a acomplex number . Example : 5+3i  - (3+3i) = 2 is not acomplex number.

Product of 2 complex number need not be a complex number.

example:  3+i and 3-i  has the product (3+i)(3-i) = 3^2 -i^2 = 9- (-1) = 10  is not a complex number.

william1941 | College Teacher | (Level 3) Valedictorian

Posted on

Let us take 2 complex numbers a+ bi and c+ di.

Now the sum of a+ bi and c+ di is (a+c) + (b+d)i , which is a complex number (as you can see a, b, c and d are different)

The difference of the two complex numbers a+ bi –( c+ di )= (a-c) + (b-d)i which is a complex number

The product of a+ bi and c+ di is (a+ bi)*(c+ di) = ac + (ad+cb)i –bd which is also a complex number

The inverse of a+bi = 1/(a+bi)= a-bi/(a+bi)(a-bi) = a-bi/(a^2 + b^2) which is also a complex number.

So all of the given options are complex numbers.