x² + c is actually a quadratic function.
And x² + c = 0, it usually has two zeros which are solutions.
But for when c = 0,
x² + c = 0
x² + 0 = 0
x² = 0
Taking the square root of both sides.
x = 0. Here it only has one zero.
So the function x² + c, only has one root for c = 0.
