Question

Two complex numbers are represented by c + di and , where c, d, e, and f are positive real numbers. In what two quadrants may the product of these complex numbers lie? Explain your answer in complete sentences.

Answer 1

Answer: In quadrant 1 or quadrant 2.

Explanation:

We have the numbers:

x = c + di

y = e + fi

where c, d, e and f are real positive numbers.

the product of these numbers is:

x*y = (c + di)*(e + fi) = c*e + c*fi + d*ei ´+d*f*i^2

x*y = c*e - d*f + (c*f + d*e)i

where I used that i^2 = -1

knowing that c,f, d and e are positive numbers, then the imaginary part of the product must be always positive.

For the real part, we have c*e - d*f, that can be positive o negative depending on the values of c, e, d, and f.

So we have that the product must lie always in one of the upper two quadrants, quadrant 1 or quadrant 2 because the imaginary part is always positive and the real part can be positive or negative.