If 'veca.veca=0' and 'veca. vecb=0', then what can be concluded about the vector 'vecb' ?

If |veca|=2,|vecb|=3 and veca.vecb=4 , then |veca-vecb| is equal to

If veca,vecb,veca+vecb are unit vectors, then prove that the angle between veca and vecb is (2pi)/(3)

Let 'veca' and 'vecb' be two unit vectors and 'theta' is the angle between them. Then 'veca,+vecb' is a unit vector if

If veca + vecb + vecc = 0, |veca| = 3, |vecb| = 5 and |vecc| = 7, then the angle between veca and vecb is : a) pi/3 b) pi/2 c) cos ^(-1) (( 2)/(225)) d) (pi)/(4)