@Kitonum Unfortunately, in this case, variables will be inserted into the expression if they are previously defined. It is necessary f3: = (x, y) ->.

@acer Thanks for explaining!

@tomleslie Thank you, your approach is perfect!

@acer Thanks for help!

Tell me why on the sides of the recesses when constructing this graph?

@mmcdara Thanks for help!

@Kitonum  I'm trying to do it this way

F := x^2+y^2 = 1; 
FisInAreaIncludeBorders := lhs(F) <= rhs(F); 
isInAreaIncludeBorders := proc (x::float, y::float) 
if FisInAreaIncludeBorders then return 'true' else return 'false' end if 
end proc; 
isInAreaIncludeBorders(0.5, 0.5)