Determine all real numbers a such that the equation x2−(5−a)x+a2−11a−46=0 has two real solutions, one of which is less than 2, and the other greater than 2.
Solution
Let the roots of the quadratic be x1 and x2, with x1<2<x2.
The quadratic equation is x2−(5−a)x+a2−11a−46=0.
By Vieta's formulas: - x1+x2=5−a - x1x2=a2−11a−46
Since the equation must have two real solutions, the discriminant must be non-negative: D = [-(5-a)] 2 - 4(a 2 - 11a - 46) = (5-a) 2 - 4(a 2 - 11a - 46) Expand: (5−a)2=25−10a+a2 So, D=25−10a+a2−4(a2−11a−46) =25−10a+a2−4a2+44a+184 =(25+184)+(−10a+44a)+(a2−4a2) =209+34a−3a2 So, D≥0: 209+34a−3a2≥0
Now, the roots must be on opposite sides of 2. For a quadratic ax2+bx+c=0, the value at x=2 is: f(2)=(2)2−(5−a)(2)+a2−11a−46 =4−2(5−a)+a2−11a−46 =4−10+2a+a2−11a−46 =(4−10−46)+(2a−11a)+a2 =(−52)−9a+a2 =a2−9a−52
For the roots to be on opposite sides of 2, f(2) must be negative (since the parabola opens upwards): f(2)<0⟹a2−9a−52<0
So, we have two inequalities: 1. 209+34a−3a2≥0 2. a2−9a−52<0
Let's solve each.
First inequality: 209+34a−3a2≥0 −3a2+34a+209≥0 3a2−34a−209≤0 Find the roots: a=2⋅334±(−34)2−4⋅3⋅(−209) =634±1156+2508 =634±3664 3664≈60.5 So, a1=634+60.5≈694.5≈15.75 a2=634−60.5≈6−26.5≈−4.42 So, 3a2−34a−209≤0 for −4.42≤a≤15.75.
Second inequality: a2−9a−52<0 Find the roots: a=29±81+208 =29±289 =29±17 So, a1=29+17=13 a_2 = 9 - 17}{2} = -4 So, a2−9a−52<0 for −4<a<13.
The intersection of the two intervals: −4.42≤a≤15.75 and −4<a<13
So, a must satisfy −4<a<13.
Final answer: All real numbers a such that −4<a<13.
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