Midline(redirected from midlines)
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(1) A midline of a triangle is the line segment joining the midpoints of two sides of the triangle. The midline is parallel to the third side, or base, of the triangle. The length of the midline is one-half that of the base. The areas of the two parts into which the midline divides the triangle are in the ratio 1:3.
(2) The midline, or median, of a trapezoid is the line segment joining the midpoints of the nonparallel sides of the trapezoid. The midline is parallel to the bases of the trapezoid, and its length is one-half the sum of the lengths of the bases.