There's this easy question that's been bugging me all day long and I would be very thankful if somebody could tell me what I'm doing wrong when attempting to solve it.

The problem goes: 'Two real numbers are chosen at random from the interval $I = (-1,1]$. If it is known that their sum is a negative real number, what is the probability that the first number is larger than $1/2$ and at the same time, its square is smaller than the second number.'

What I've tried to do so far:

> 
> * Let A be the event: 'The first number is larger than 1/2; i.e $x>1/2$'.
> * Let B be the event: 'The square of the first number is smaller than the second number; i.e $x^2 < y$'
> * Let C be the event: 'The sum of the two numbers is a negative real number; i.e $x+y<0$'
> 
> 
> We're trying to find $p((A\*B)|C)$, which, if I got this right would be
>  $p(A\*B\*C)/p(C)$. However, unless I'm mistaken, this won't work. I
>  tried to draw a geometrical representation using Cartesian
>  coordinates, but so far I only got that there's no way that A and
>  B can be true if C is also true.
> 
> 
> 

Any insight on what I'm doing wrong would be greatly appreciated.