Continuous random variable={-b-(b^2-4ac)^.5}/(2a)=x?

1. Apr 19, 2007

inv

[Solved]Continuous random variable={-b-(b^2-4ac)^.5}/(2a)=x?

1. The problem statement, all variables and given/known data
A factory is supplied with grain at the beginning of ea week.The weekly demand,X thousand tonnes for grain from this factory is a continuous random variable having the probability density function given by
f(x)=2(1-x),0<x<1
0 ,otherwise
Find the quantity of grain in tonnes the factory should've in stock in the beginning of a week in order to be 98% certain that the demand in that week will be met.

*I've found 2 answers but the answer sheet only chose 1 of 'em,found by using the (-b-(b^2-4ac)^.5)/(2a)

2. Relevant equations
(-b+(b^2-4ac)^.5)/(2a)

3. The attempt at a solution

I've integrated f(x) with the lower x value and higher x value 0 and 1 respectively and got:
[2x-x^2]=0.98
0=x^2-2x+0.98
(-b+(b^2-4ac)^.5)/(2a)
x=1.14 and 0.859
*I don't know why the answer sheet chose only the 0.859,why?

Last edited: Apr 19, 2007
2. Apr 19, 2007

cristo

Staff Emeritus
What's the domain of f in the orginal question?

3. Apr 19, 2007

inv

Domain of f is x,domain is x and range is y.That's what u want?

4. Apr 19, 2007

cristo

Staff Emeritus
No, sorry, I'll rephrase my question. The function f is only defined for certain values of x. What are these? Are both of your answers in the interval of allowed x values?

5. Apr 19, 2007

inv

Nnly 1 of 'em're,which makes this case solved.Tq so much for making me realize.

6. Apr 19, 2007

HallsofIvy

Staff Emeritus
No, the domain of a function is the set of possible values for x- and those are given in the problem.

7. Apr 19, 2007

inv

Thanks for the reply a bunch Ivy ,bye.