Question 983801
Hi dear!
Lets draw this analogy;
consider call cost and call time.
let x be call time,y be call time.
It is noted that call cost  depends on call time. i.e y=x
now for any value of x,
y will have a "unique"value.Also y,x will have a value.
if we say,let <prev>
{{{y=sqrt(x)}}} you will notice that for any value of x,say 9,y will not have a unique value but two values +3 or -3 which is not an idea of a fuction but a rule.
In conclusion,we say that a fuction f(x) must have a unique value.