Find the polynomial g(x) with integer coefficients of least degree (and positive leading coefficients as small as possible) for which 1-2i is a double root?

1 Answer
Oct 5, 2017

g(x)=x^4-4x^3+14x^2-20x+25

Explanation:

As the polynomial g(x) has integer coefficients and 1-2i is a double root of the polynomial, it has its factor (x-(1-2i))^2.

But it will give us complex coefficients and hence for integers as coefficients we must have another double root which is complex conjugate of 1-2i i.e. 1+2i. Hence (x-(1+2i))^2 too is a factor.

Hence the polynomial with least degree would be

g(x)=(x-(1-2i))^2(x-(1+2i))^2

= ((x-1+2i)(x-1-2i))^2

= (x^2-xcolor(red)(-2ix)-x+1color(red)(+2i+2ix-2i)-4i^2)^2

= (x^2-2x+1+4)^2

= (x^2-2x+5)^2

= x^4+4x^2+25-4x^3+10x^2-20x

(using (a+b+c)^2=a^2+b^2+c^2+2ab+2ac+2ab)

= x^4-4x^3+14x^2-20x+25