SOLUTION: For the quadratic equation x^2 + kx +7 =0, if 3+sqrt2 is a root, what is the value of k ? I said 6 but my teacher marked it wrong. Please help me :)

Algebra ->  Quadratic Equations and Parabolas  -> Quadratic Equations Lessons  -> Quadratic Equation Lesson -> SOLUTION: For the quadratic equation x^2 + kx +7 =0, if 3+sqrt2 is a root, what is the value of k ? I said 6 but my teacher marked it wrong. Please help me :)      Log On


   

Question 118532: For the quadratic equation x^2 + kx +7 =0, if 3+sqrt2 is a root, what is the value of k ?
I said 6 but my teacher marked it wrong. Please help me :)
Answer by tutor_paul(519) About Me  (Show Source):
You can put this solution on YOUR website!
if something is a "root," it means it is a value for x which satisfies the equation. So plug the root in for x ans solve for k:
x%5E2%2Bkx%2B7=0
%283%2Bsqrt%282%29%29%5E2%2Bk%283%2Bsqrt%282%29%29%2B7=0
9%2B6sqrt%282%29%2B2%2Bk%283%2Bsqrt%282%29%29%2B7=0
9%2B8.485%2B2%2Bk%284.414%29%2B7=0
k%284.414%29%2B26.485=0
k%284.414%29=-26.485
highlight%28k=-6%29