TeX source:

\vec{a}= \frac{dv_x}{dt} \widehat{x}+ \frac{dv_y}{dt} \widehat{y}+ \frac{dv_z}{dt} \widehat{z}=a_x \widehat{x}+a_y\widehat{y} +a_z\widehat{z}