Engineering Board Exam Reviewer · Quadratic Equations · hard
If one root of ax2+bx+c=0 is twice the other, which relation among the coefficients must hold?
- 2b2=9ac
- b2=9ac
- 2b2=ac
- b2=2ac
Answer
A. 2b2=9ac
Solution
Let the roots be r and 2r. Then 3r=- ba and 2r2= ca. From the first, r=-b/3a, so 2·b2/9a2= ca 2b2=9ac.
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