Page:Calculus Made Easy.pdf/107

assigned value of $x$, 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

Fig. 25.

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