Probablgebra!!

x 2 + p x + ( p + 2 ) 4 = 0 x^{2}+px+\frac{(p+2)}{4} = 0

If p p is chosen at random in the closed interval [ 0 , 5 ] [0,5] , the probability that the above equation has real roots can be written as A B \frac{ A } { B } where A A and B B are coprime positive integers. Find A + B A + B .


The answer is 8.

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2 solutions

Tran Hieu
Jan 21, 2016

The first equation could be rewrite as ( x + p 2 ) 2 = p 2 4 p 4 1 4 (x+\frac{p}{2})^2 = \frac{p^2}{4}-\frac{p}{4}-\frac{1}{4} .

In order for this to have real roots, the RHS must be non negative. Apply that and rewrite we have

( p 2 1 4 ) 2 9 16 (\frac{p}{2}-\frac{1}{4})^2 \geq \frac{9}{16}

or p 2 1 4 3 4 or p 2 \frac{p}{2}-\frac{1}{4}\geq\frac{3}{4}\text{ or }p\geq2

Because we choose p random from [0,5] so we have probability of p 2 is 5 2 5 so our answer is 8 p\geq2\text{ is } \frac{5-2}{5}\text{ so our answer is }\boxed{8}

Note that there is a small case where 1 4 p 2 3 4 \frac{1}{4}-\frac{p}{2}\geq\frac{3}{4} , but that could not be satisfy with p in [0,5]

Exact and elegant solution

Mohit Gupta - 5 years, 4 months ago
Prince Loomba
Jan 26, 2016

I also did the same b^2-4ac>=0... p>=2 or p <= -1. Thus 2-5 is the answer.
Probability (5-2)/5=3/5... answer=3+5=8

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