If is chosen at random in the closed interval , the probability that the above equation has real roots can be written as where and are coprime positive integers. Find .
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The first equation could be rewrite as ( x + 2 p ) 2 = 4 p 2 − 4 p − 4 1 .
In order for this to have real roots, the RHS must be non negative. Apply that and rewrite we have
( 2 p − 4 1 ) 2 ≥ 1 6 9
or 2 p − 4 1 ≥ 4 3 or p ≥ 2
Because we choose p random from [0,5] so we have probability of p ≥ 2 is 5 5 − 2 so our answer is 8
Note that there is a small case where 4 1 − 2 p ≥ 4 3 , but that could not be satisfy with p in [0,5]