Easy points are not easy to get

Algebra Level 3

Known kx²+3x-1=0 have real solution, then what is the smallest possible value for k?


The answer is -2.25.

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2 solutions

Dawar Husain
May 20, 2015

x = 3 ± 9 + 4 k 2 k x=\dfrac { -3\pm \sqrt { 9+4k } }{ 2k } For this to be real, the expression inside roots must be 0 \geq 0 . Therefore,

9 + 4 k 0 9+4k \geq 0

k + 9 4 0 k + \dfrac{9}{4} \geq 0

k [ 9 4 , ] \therefore k \in \left[-\dfrac{9}{4}, \infty\right] Least Value = 9 4 = 2.25 \implies \text{Least Value = }-\dfrac{9}{4} = \boxed{\large{-2.25}}

quadratic equation have real solution if D=b^2 - 4ac >=0 (not negatif)

kx^2+3x-1 = 0 => a=k, b=3, and c=-1

D=b^2-4ac=3^2 - 4 k (-1)=9+4k

D>=0

9+4k>=0

k>=(-9)/4

k>=-2,25

so, small value for k is -2,25

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