When approximating an area with rectangles, a {{CMD_80}} is the {{MATH_81}}-coordinate that determines the height of {{MATH_82}} rectangle. For {{MATH_83}}, we denote a sample point as: {{MATH_84}} which lies in the subinterval {{MATH_85}} and the value {{MATH_86}} is the height of the {{MATH_87}} rectangle. {{CMD_88}}