A compound formed by elements `A` and `B` crystallises in a cubic structure where `A` atoms are present at the corners of a cube and the `B` atoms are present at the face centres.The formula of the compound is

Explanation

An atom at the corner of the cubic contributes `1//8` to the unit cell. Hence, number of atoms of A in the unit cell `=8xx(1)/(8) =1`
At atoms at the face of the cube contributes `(1)/(2)` to the unit cell. Hence, number of atoms B in the unit cell `=6xx(1)/(2)=3`
Thus, the formula is `AB_(3)`.