SOLUTION: find the value of k that makes the sum of the roots of 3x^2-(3k+2)x+18=0 equal to 6.

Algebra ->  Quadratic Equations and Parabolas  -> Quadratic Equations Lessons  -> Quadratic Equation Lesson -> SOLUTION: find the value of k that makes the sum of the roots of 3x^2-(3k+2)x+18=0 equal to 6.      Log On


   



Question 891093: find the value of k that makes the sum of the roots of 3x^2-(3k+2)x+18=0 equal to 6.
Answer by Edwin McCravy(20054) About Me  (Show Source):
You can put this solution on YOUR website!
3x%5E2-%283k%2B2%29x%2B18=0

We make the left side into a "monic" polynomial.  A "monic" polynomial
is one with the coefficient of the term with the greatest degree as 1.
So we divide each coefficient by 3:

%283%2F3%29x%5E2-%28%283k%2B2%29%2F3%29x%2B%2818%2F3%29=%280%2F3%29

x%5E2-%28%283k%2B2%29%2F3%29x%2B6=0

In a "monic" polynomial of degree n, the coefficient of the term in
xn-1 is -1 times the sum of the roots of the polynomial.

So the sum of the roots is -1%2Aexpr%28-%283k%2B2%29%2F3%29 = %283k%2B2%29%2F3

Set this equal to 6:

%283k%2B2%29%2F3%22%22=%22%226

 3k%2B2%22%22=%22%2218

3k%22%22=%22%2216

k%22%22=%22%2216%2F3

Edwin