A complex number in its rectangular form is expressed as x+iy where x is the real part of the complex number and y is the imaginary because it is attached to the imaginary number i.
Given the complex number 4-5i, comparing the complex number with x+iy
4 = x and iy = -5i
Hence x = 4 and y = -5.
Since x is the real part of the complex number x+iy,<em> hence 4 will be the real part of the complex number 4-5i based on comparison. </em>
