\( d(P_1, P_2 ) = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2} \)
\( f' (x) = \mathop {\lim }\limits_{h \to 0} \frac{{f\left( x + h \right) - f\left( x \right)}}{{h}} \)