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What is the sum of the square of the roots of x^2 - 6x + 8 = 0

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grade81 | Student, Kindergarten | eNoter

Posted July 2, 2013 at 2:02 PM via web

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What is the sum of the square of the roots of x^2 - 6x + 8 = 0

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted July 2, 2013 at 2:05 PM (Answer #1)

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The quadratic equation given is x^2 - 6x + 8 = 0.

x^2 - 6x + 8 = 0

=> x^2 - 4x - 2x + 8 = 0

=> x(x - 4) - 2(x - 4) = 0

=> (x - 2)(x - 4) = 0

=> x = 2 and x = 4

The sum of the squares of the roots is 2^2 + 4^2 = 4 + 16 = 20

The required sum is 20.

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degeneratecircle | High School Teacher | (Level 2) Associate Educator

Posted July 2, 2013 at 2:43 PM (Answer #2)

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There's a method that doesn't require you to find the roots themselves. Call the unknown roots `r_1,r_2.` We know that the quadratic factors as

`x^2-6x+8=(x-r_1)(x-r_2),` and expanding the right side gives

`x^2-6x+8=x^2-(r_1+r_2)x+r_1r_2.` Equating coefficients gives

`r_1+r_2=6`                   (1)

`r_1r_2=8`                      (2)

Multiply equation (1) by `r_1` and then `r_2` to get the new equations

`r_1^2+r_1r_2=6r_1`           (3)

`r_1r_2+r_2^2=6r_2`           (4)

Add (3) and (4) to get `r_1^2+r_2^2+2r_1r_2=6(r_1+r_2).`

Now use (1) and (2) to get `r_1^2+r_2^2+16=36,` so we get the answer `r_1^2+r_2^2=20.`

For this particular problem, this method takes longer. However, it can be used to solve similar problems where solving for the roots is tedious.

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