1. Find angle(A,B)
`A=(3,1,1)`,`B=(-1,2,1)`

Solution:
Here `vec A=(3,1,1),vec B=(-1,2,1)`

The angle between two vectors `vec A` and `vec B` is given by

`cos(theta)=(vec A * vec B)/(|vec A| * |vec B|)`

1. Calculate dot product
`vec A * vec B`

`=A_1*B_1 + A_2*B_2 + A_3*B_3`

`=3*(-1) + 1*2 + 1*1`

`=-3 + 2 + 1`

`=0`

2. Calculate magnitude of vectors
`|vec A|`

`=sqrt(A_1^2 + A_2^2 + A_3^2)`

`=sqrt(3^2 + 1^2 + 1^2)`

`=sqrt(9 + 1 + 1)`

`=sqrt(11)`

`=3.3166`


`|vec B|`

`=sqrt(B_1^2 + B_2^2 + B_3^2)`

`=sqrt((-1)^2 + 2^2 + 1^2)`

`=sqrt(1 + 4 + 1)`

`=sqrt(6)`

`=2.4495`


3. Calculate angle between vectors
`cos(theta)=(0)/(3.3166 * 2.4495)=0`

`:. theta=cos^(-1)(0)=1.5708` rad

`:. theta=90 ^circ`