Question 699378
{{{x+y=9}}} this is linear equation, graph is a line, no {{{x-values}}} repeating; so, it {{{IS}}} a function

{{{ graph( 600, 600, -10, 10, -10, 10,-x+9) }}}

and

{{{x^2+y^2=1}}}...here you have a circle



{{{drawing( 600, 600, -5, 5, -5, 5,
  grid( 1 ),
  circle( 0, 0, 1 )
  
)}}}

In any Cartesian coordinate system,a circle does not correspond to a function.