SOLUTION: If the two roots of the quadratic 7x^2+3x+k$ are (-3張sqrt(299))/14, what is k?

Algebra ->  Quadratic Equations and Parabolas  -> Quadratic Equations Lessons  -> Quadratic Equation Lesson -> SOLUTION: If the two roots of the quadratic 7x^2+3x+k$ are (-3張sqrt(299))/14, what is k?      Log On


   



Question 1086048: If the two roots of the quadratic 7x^2+3x+k$ are (-3張sqrt(299))/14, what is k?
Found 2 solutions by ikleyn, MathTherapy:
Answer by ikleyn(52787) About Me  (Show Source):
You can put this solution on YOUR website!
.
k%2F7 is the product of the roots (Vieta's theorem):

k%2F7 = %28%28-3%2Bi%2Asqrt%28299%29%29%2F14%29%2A%28%28-3-i%2Asqrt%28299%29%29%2F14%29 = %283%5E2%2B299%29%2F14%5E2 = 308%2F14%5E2.


Hence, k = %28308%2A7%29%2F14%5E2 = 11.


Thanks to @MathTherapy for pointing my typo in my previous version.


Answer by MathTherapy(10552) About Me  (Show Source):
You can put this solution on YOUR website!

If the two roots of the quadratic 7x^2+3x+k$ are (-3張sqrt(299))/14, what is k?
Product of the roots = matrix%281%2C3%2C+c%2Fa%2C+or%2C+k%2F7%29
Roots: %28-+3+%2B-+i+%2A+sqrt%28299%29%29%2F14 =====> %28-+3+%2B+i+%2A+sqrt%28299%29%29%2F14 and %28-+3+-+i+%2A+sqrt%28299%29%29%2F14
We then get: %28-+3+%2B+i+%2A+sqrt%28299%29%29%2F14+%2A+%28-+3+-+i+%2A+sqrt%28299%29%29%2F14+=+k%2F7
Reducing, we get: 308%2F14%5E2+=+k%2F7
14%5E2k+=+7%28308%29 ------ Cross-multiplying