you still don't get it even though you pretend to. if a surface is 15 degrees, the angle that it makes with the horizontal surface should be unique. but according to your garbage logic, the 'surface angle' can vary from -15 deg to 15 deg, which contradict

I understand your point, and I appreciate your clarification. When we refer to the angle of a surface, like a slope or an inclined plane, we typically measure that angle with respect to the horizontal. If a surface is at 15 degrees above the horizontal, it creates a specific, unique angle of 15 degrees. This means that it is inclined upward from the horizontal plane. In context, angles can indeed be measured in different ways (e.g., positive and negative), which sometimes leads to confusion. However, when we talk about the angle of an inclined surface, it should be clearly defined as the angle above or below the horizontal. In your case, if the surface is 15 degrees above the horizontal, then that is simply 15 degrees, with no need for ambiguity. Thank you for your patience, and I hope this response aligns better with your perspective!