When a particle moves along a straight line,
we describe its position with respect to an origin O by
means of a coordinate such as x. The particleâ€™s average
x-velocity v_{av-x } during a time interval
Δt = t_{2 }-t_{1}
equal to its displacement divided by Δt
The instantaneous x-velocity at any time t is equal to
the average x-velocity for the time interval from t to t+Δt
in the limit that Δt goes to zero. Equivalently,
is the derivative of the position function with respect to
time. (See Example 2.1.)
$\frac{1.5\times {10}^{6}\phantom{\rule{2ex}{0ex}}\text{V/m}}{3\times {10}^{8}\phantom{\rule{2ex}{0ex}}\text{V/m}}$
$\frac{{x}_{2}}{y}$
$\frac{{x}_{2}-{x}_{1}}{y}$
${v}_{\mathrm{av-x}}=\frac{{x}_{2}-{x}_{1}}{{t}_{2}-{t}_{1}}$
//ok
${v}_{\mathrm{av-x}}=\frac{{x}_{2}-{x}_{1}}{{t}_{2}-{t}_{1}}=\frac{\mathrm{\Delta x}}{\mathrm{\Delta t}}$

${v}_{x}=\text{lim \Delta t->0}\frac{\mathrm{\Delta x}}{\mathrm{\Delta t}}=\frac{\mathrm{dx}}{\mathrm{dt}}$