- #1
LiHJ
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Homework Statement
Dear Mentors and Helpers,
Here's the question:
Find the possible values of k such that one root of the equation 2x^2 + kx + 9 = 0 is twice the other.
Homework Equations
My classmate's working:
Discriminate > 0
k^2 - (4)(2)(9) > 0
k^2 -72 > 0
[k + sqrt (72)] [k- sqrt(72)] > 0
Answer: k > sqrt (72) or k < - sqrt(72)
The Attempt at a Solution
My working:
Let p and 2p be the roots of the equation.
Sum of roots:
3p = (-k)/2
p = (-k)/6 -----(1)
Product of roots:
2p^2 = 9/2 ------(2)
Substitute (1) into (2):
2(-k/6)^2 = 9/2
(k^2)/36 = 9/4
k^2 = 81
k = +/- (9)
Answer: +/- (9)
Dear Mentors and Helpers,
Please help me to check whether my friends working is correct or mine is correct. Thanks for your time.
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