You never know when set notation is going to pop up. Usually, you'll see it when you learn about solving inequalities, because for some reason saying x < 3 isn't good enough, so instead they'll want you to phrase the answer as the solution set is { x | x is a real number and x < 3 }.
If the skin really looks like the color of ash, a deeper more persistent gray or gray-blue color, this could potentially be due to a side effect of certain medications, with or without interaction from sunlight.
Cyanosis occurs when thereâs too little oxygen in the blood. Oxygen-rich blood is deep red and is the cause of your skinâs normal color. Under oxygenated blood is more blue and causes your skin to be bluish purple. Cyanosis can develop quickly as a result of an acute health problem or external factor.