A journey towards infinity

Algebra Level 2

If both real roots of the equation x 2 ( a 3 ) x + a = 0 x^2-(a-3)x+a=0 , where a a is real, are greater than 2 2 , what is the range of a a ?

[ 7 , 7,\infty ) [9,10) [7,9) [7,9]

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1 solution

Chew-Seong Cheong
Jul 23, 2019

Solving the quadratic, x 2 ( a 3 ) x + a = 0 x^2 - (a-3)x + a = 0 x = a 3 ± ( a 3 ) 2 4 a 2 \implies x = \dfrac {a-3 \pm \sqrt{(a-3)^2 -4a}}2 . For the roots to be real,

( a 3 ) 2 4 a 0 a 2 10 a + 9 0 ( a 1 ) ( a 9 ) 0 a 4 a 9 \begin{aligned} \implies (a-3)^2 - 4a & \ge 0 \\ a^2 - 10a + 9 & \ge 0 \\ (a-1)(a-9) & \ge 0 \\ \implies a \le 4 & \cup a \ge 9 \end{aligned}

For both roots to be > 2 > 2 ,

a 3 ( a 3 ) 2 4 a 2 > 2 a 3 a 2 10 a + 9 > 4 a 7 > a 2 10 a + 9 Squaring both sides a 2 14 a + 40 > a 2 10 a + 9 a < 10 \begin{aligned} \frac {a-3 - \sqrt{(a-3)^2 -4a}}2 & > 2 \\ a-3 - \sqrt{a^2-10a+9} & > 4 \\ a-7 & > \sqrt{a^2-10a+9} & \small \color{#3D99F6} \text{Squaring both sides} \\ a^2 - 14a + 40 & > a^2-10a+9 \\ \implies a < 10 \end{aligned}

Therefore, for both roots to be > 2 >2 , a [ 9 , 10 ) a \in \boxed{[9, 10)} .

This was really easy, to be honest, make them harder

Neymar Jr - 1 year, 8 months ago

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