assigned value of ${\displaystyle x}$, the height of the ordinate in the second curve is proportional to the slope of the first curve.

Fig. 23.	Fig. 24.

If a curve comes to a sudden cusp, as in Fig. 25, the slope at that point suddenly changes from a slope

upward to a slope downward. In that case ${\displaystyle {\dfrac {dy}{dx}}}$ will clearly undergo an abrupt change from a positive to a negative value.