In geometry, a base is a side of a plane figure or face of a solid, particularly one perpendicular to the direction height is measured or on what is considered to the bottom of the object. This usage can be applied to a triangle, parallelogram, trapezoid, cylinder, cone, pyramid, parallelopiped or frustum. By extension, the length or area of a figure's base is also called its base. As such, bases are commonly used in formulas for area and volume. For example, a formula for the area of a triangle is (1/2) × base × height. Another example is an octogonal prism; its base is an octogon, and its volume is the area of the base times the height.
- Palmer, C.I.; Taylor, D.P. (1918). Plane Geometry. Scott, Foresman & Co. pp. 38, 315, 353.
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