In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles.〔Wentworth p. 8〕 As a rotation, a right angle corresponds to a quarter turn (that is, a quarter of a full circle).〔Wentworth p. 11〕
Closely related and important geometrical concepts are perpendicular lines, meaning lines that form right angles at their point of intersection, and orthogonality, which is the property of forming right angles, usually applied to vectors. The presence of a right angle in a triangle is the defining factor for right triangles,〔Wentworth p. 40〕 making the right angles basic to trigonometry.
The term is a calque of Latin ''angulus rectus''; here ''rectus'' means "upright", referring to the vertical perpendicular to a horizontal base line.
== Symbols ==
In Unicode, the symbol for a right angle is .〔Unicode 5.2 Character Code Charts (Mathematical Operators ), (Miscellaneous Mathematical Symbols-B )〕
In diagrams the fact that an angle is a right angle is usually expressed by adding small right angle that forms a square with the angle in the diagram, as seen the diagram of a right triangle to the right. The symbol for a measured angle, an arc with a dot, is used in some European countries, including German-speaking countries and Poland, as an alternative symbol for a right angle.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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