Question

If vector r×dr = 0, then show that vector r is constant.

Answer 1

We have,

$r.dr = 0$
Then,
$= > 2r.dr = 0 \\ = > r.dr + dr.r = 0 \\ = > d(r.r) = 0 \\ = > { |r| }^{2} = constant \: \\ = > r \: = constant \:$
Hence Proved.

Answer 2

Answer:

plz explain more.......this and how 2r.dr came