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A and C are points in an argand diagram representing the complex ...

A and C already represent 2 points of the square and since they are diagonally ... So B should be 4+2i +(-1+3i)= 3+5i and D should be 4+2i-(-1+3i)=5-1i.

For more information, see A and C are points in an argand diagram representing the complex ...

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Complex number - Wikipedia

A complex number is a number that can be expressed in the form a + bi, where a and b are real ... has no real solution, since the square of a real number cannot be negative. ... to be purely imaginary, and the points for these numbers lie on the vertical axis of the complex plane. ... For example, 2 + 3i is a complex number.

For more information, see Complex number - Wikipedia

If point A = 2+i ; B = 4+3i ; C = 2+5I ; D = 3i

A  = (2,1) B = (4,3), C = (2,5), D = (0,3)

Using the distance formula  \sqrt{({x_{2} - x_{1}}){^2}+({y_{2} - y_{1}}){^2}} between AB BC, CD, DA

AB  = \sqrt{({4-2}){^2}+({3-1}){^2}} = 4

BC = ​​\sqrt{({2-4}){^2}+({5-3}){^2}} = 4

CD = \sqrt{({0-2}){^2}+({3-5}){^2}} =4

DA = \sqrt{({0-2}){^2}+({3-1}){^2}} = 4