the answer: to find the angle between the two vectors, the following method can be used:
first, computing the scalar product between the two vectors A and B, after, their length. the next is to use the main formula cos T = A*B / //A// //B//, where A*B is scalar product
T= teta
Practice: A*B = (2, 1,-4) * (-3, 0,1) =(2x-3)+(1x0)+(-4x1)=-6-4= -10
//A// = sqrt( 2² + 1² +4²) = sqrt(4+1+16) = sqrt(21)=4.58
//B// = sqrt(10)=3.16
so, cosT = -10 / 3.16*4.58 = -10/14.48 = -0.69, cosT = -0.69, therefore, T= arccos(-0.69)=46,33°
