vector derivative Definition and 1 Threads

Drawing : I draw a diagram explaining the situation to the right. The vectors ##\mathbf{A}## and ##\mathbf{B}## are drawn at some time ##t## making an angle ##\theta## between them. At some later time ##t+dt##, the same vectors have changed to ##\mathbf{A}(t+dt)## and ##\mathbf{B}(t+dt)## while...