While both retinol and Retin-A contain retinoids, they differ in their concentration. Retinol and other forms, such as retinal and retinyl palmitate are found in many over-the-counter anti-aging creams. While these all contain a form of vitamin A, the concentration of retinoids is weak. Retin-A is a prescription retinoid and is a brand name for tretinoin, which is the carboxylic acid form of vitamin A and was one of the early topical acne treatments. Retin-A is sold by prescription because its acid composition is much greater than in over-the-counter products.
LINE (an undefined term) In geometry, a line has no thickness but its length extends in one dimension and goes on forever in both directions. A line is depicted to be a straight line with two arrowheads indicating that the line extends without end in two directions.
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