Question 238636
In order to be a function, each {{{x}}} must match with a single
{{{f(x)}}}. In the 1st case, {{{f(x) = x^2}}}, and there can only be 
one {{{x}}} for each {{{f(x)}}}.
In the 2nd case, {{{ f(x) = + sqrt(x)}}}, and {{{f(x) = -sqrt(x)}}}
Each {{{x}}} matches with 2 values of {{{f(x)}}}
Here's a graph of each. The curve on the right is not a function
{{{ graph( 500, 500, -10, 10, -10, 10, x^2, sqrt(x), -sqrt(x)) }}}