A formula that uses mathematical symbols to express the relationship between several quantities in natural sciences such as mathematics, physics, chemistry and biology. It is universal and applicable to all similar problems. In mathematical logic, a formula is a formal grammatical object to express a proposition, but the proposition may depend on the free variable value of the formula.
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In geometry, an angle is a geometric object composed of two rays with a common endpoint. These two rays are called the edges of an angle, and their common endpoint is called the vertex of the angle. The general angle is assumed to be in Euclidean plane, but it can also be defined in Euclidean geometry. Angle is widely used in geometry and trigonometry.
Euclid, the father of geometry, once defined an angle as the relative inclination of two non-parallel straight lines in a plane. Proclos thinks that angle may be a trait, a quantifiable quantity, or a relationship. Oldham thinks that an angle is a deviation from a straight line, and Cabus of Antioch thinks that an angle is a space between two intersecting straight lines. Euclid thinks that an angle is a relationship, but his definitions of right angle, acute angle and obtuse angle are all quantitative.