<p>If 2x^2 +kx +9=0 has a double root, which of the following is a possible value of k?
1)-8.49
2)2.12
3)5.61
4)9.25
5)72</p>
<p>What is the answer and What does he mean by a double root here??</p>
<p>Double root, I assume, means that there is only one answer/solution/zero/root/factor to the equation, aka the one solution has a multiplicity of 2.</p>
<p>The easiest way to do this is to set the discriminant equal to zero.</p>
<p>root (b^2 - 4ac) = 0
root (k^2 - 4(2)(9)) = 0
k^2 - 72 = 0
k = +/- root(72) = +/- 8.49</p>