If [c,d] is a subinterval of [a,b] under one of the left-endpoint sum rectangles, then the area of the rectangle contributing to the left-endpoint estimate is f(c)(d−c). But, f(c)≤f(x) for c≤x≤d, so the area under the graph of f between c and d is f(c)(d−c) plus the area below the graph of f but above the horizontal line segment at height f(c), which is positive. As this is true for each left-endpoint sum interval, it follows that the left Riemann sum is less than or equal to the area below the graph of f on [a,b].