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# polynomial

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Find a monic quartic polynomial  with rational coefficients whose roots include 2+sqrt(2) and 1-sqrt(5) . Give your answer in expanded form.

Jan 28, 2021

#1
+8456
+2

The coefficients are rational => Sum of roots and product of roots are also rational.

=> conjugates of 2 + sqrt 2 and 1 - sqrt 5 must also be roots of the polynomial.

=> The roots are 2 + sqrt 2, 2 - sqrt 2, 1 - sqrt 5, 1 + sqrt 5.

The required monic quartic polynomial is

$$\quad(x - (2 + \sqrt 2))(x - (2 - \sqrt 2))(x - (1 - \sqrt 5))(x - (1 + \sqrt 5))\\ =(x^2 - 4x + 2)(x^2 - 2x - 4)\\ =x^4 - 6x^3 +6x^2 +12x-8$$

Jan 28, 2021

#1
+8456
+2

The coefficients are rational => Sum of roots and product of roots are also rational.

=> conjugates of 2 + sqrt 2 and 1 - sqrt 5 must also be roots of the polynomial.

=> The roots are 2 + sqrt 2, 2 - sqrt 2, 1 - sqrt 5, 1 + sqrt 5.

The required monic quartic polynomial is

$$\quad(x - (2 + \sqrt 2))(x - (2 - \sqrt 2))(x - (1 - \sqrt 5))(x - (1 + \sqrt 5))\\ =(x^2 - 4x + 2)(x^2 - 2x - 4)\\ =x^4 - 6x^3 +6x^2 +12x-8$$

MaxWong Jan 28, 2021