First, though, here are some basic definitions. An angle is the inclination to one another of two straight lines that meet. The point at which two lines meet is called the vertex of the angle. An acute angle is less than a right angle. An obtuse angle is greater than a right angle.
Rectilinear figures are figures bounded by straight lines.
A triangle is bounded by three straight lines, a quadrilateral by four, and a polygon by more than four straight lines. A regular polygon has all its sides equal. An equilateral triangle has three equal sides. An isosceles triangle has only two equal sides. The vertex angle of a triangle is the angle opposite the base.