Isn't it true
A line can be normal and tangent to a curve
but not to a function ,
If it isn't
can you give an example ?
A tangent is a line and a normal is a line. At a specific point on a curve (or function), the normal and the tangent to that point are perpendicular to each other. So at a point on a curve (or function), the normal and tangent are not the same line. For example, for two lines to be the same, they need to be at the very least parallel to each other.