Figure 5.4 shows the relationship b

Figure 5.4 shows the relationship between TP and MP Up to point c the TP curve is increasing at an increasing rate and the return to the variable factor is increasing. MP is rising. (The slope of a tangent drawn to the TP curve would be rising .) At point c the curve is at its steepest and MP reaches a maximum. This point is referred to as the point of inflection of the curve. Beyond point c TP increases at a decreasing rate and MP falls. (The slope of a tangent would be falling.)
At point e, TP is maximized. A tangent drawn to this point has no slops and MP is therefore zero. Beyond point e MP is negative and becomes increasingly so. Economically, it makes no sense to employ workers if they have a negative effect upon production.