4 x − m 2 x 2 − 5 x + 3
How many integers m satisfy the following property:
In the domain x ∈ R − { 4 m } , the range of the expression is R .
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Tricky problem and a very nice solution!
Great solution .
@Calvin Lin why has the problem been reported by multiple members ?
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@Hung Woei Neoh , i have posted the solution
@Hung Woei Neoh @Sal Gard @Rishabh Cool , i have posted the solution .
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@Otto Bretscher @Pi Han Goh @Brian Charlesworth @Jon Haussmann @Nicolae Sapoval @Chew-Seong Cheong , please help me understand what is the error in my solution .
The issue with your solution is that quadratic discriminant has nothing to do with this.
@Pi Han Goh , can you provide an alternate solution
The phrasing of the solution could be greatly cleaned up, similar to that of the problem. I've edited the problem, and it should now be easier to understand what is being asked for. Can you edit the solution accordingly?
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Let y = 4 x − m 2 x 2 − 5 x + 3 2 x 2 − ( 4 y + 5 ) x + 3 + m y = 0 As x is real , d i s c r i m i n a n t ≥ 0 ( 4 y + 5 ) 2 − 8 ( 3 + m y ) ≥ 0 1 6 y 2 + ( 4 0 − 8 m ) y + 1 ≥ 0 A quadratic in y is non-negative for all values of y if coefficient of y 2 is positive and d i s c r i m i n a t ≤ 0 ( 4 0 − 8 m ) 2 − 4 ∗ 1 6 ∗ 1 ≤ 0 ( 5 − m ) 2 − 1 ≤ 0 ( m − 6 ) ( m − 4 ) ≤ 0 m ϵ [ 4 , 6 ] Consider m = 4 y = 4 ( x − 1 ) ( 2 x − 3 ) ( x − 1 ) y = 4 2 x − 3 , x = 1 Since x = 1 , y = 4 − 1 Hence y cannot take all real values for m = 4 . So, m = 4 is not acceptable . Similarly m = 6 is to be rejected . So for the given expression to take all real values , m ∈ ( 4 , 6 ) . There is only 1 integer in this range.