Question

what is the angle between p×q and q×p​

Answer 1

p×q=pqsin(@) n^

q×p=-pqsin(@) n^

now you think that angle between these two vectors will be 270°, since Sin(@) =-Sin(@) at 270°. But it is wrong the actual concept behind this is AT WHAT ANGLE THE VECTORS SHOULD BE IN OPPOSITE DIRECTION. Because this is cross product it is a vector and direction is necessary.

so concluding that

p×q=-q×p

since since direction is opposite hence the angle between the vectors is 180°.