If both real roots of the equation , where is real, are greater than , what is the range of ?
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Solving the quadratic, x 2 − ( a − 3 ) x + a = 0 ⟹ x = 2 a − 3 ± ( a − 3 ) 2 − 4 a . For the roots to be real,
⟹ ( a − 3 ) 2 − 4 a a 2 − 1 0 a + 9 ( a − 1 ) ( a − 9 ) ⟹ a ≤ 4 ≥ 0 ≥ 0 ≥ 0 ∪ a ≥ 9
For both roots to be > 2 ,
2 a − 3 − ( a − 3 ) 2 − 4 a a − 3 − a 2 − 1 0 a + 9 a − 7 a 2 − 1 4 a + 4 0 ⟹ a < 1 0 > 2 > 4 > a 2 − 1 0 a + 9 > a 2 − 1 0 a + 9 Squaring both sides
Therefore, for both roots to be > 2 , a ∈ [ 9 , 1 0 ) .