at 2020-10-08 src: Khan Multivar. Calculus(https://www.youtube.com/watch?v=FYMn61HLw1k&list=PLSQl0a2vh4HC5feHa6Rc5c0wbRTx56nF7&index=28) differential of a vector valued function
KWs: derivative/differential of vector valued function / position vector function