If `cos^(-1) x - cos^(-1). y/2= alpha ", then " 4x^(2) - 4xy  cos alpha + y^(2) ` is equal to

If `cos^(-1) x - cos^(-1). y/2= alpha ", then " 4x^(2) - 4xy cos alpha + y^(2) ` is equal to

If `cos^(-1) x - cos^(-1). y/2= alpha ", then " 4x^(2) - 4xy cos alpha + y^(2) ` is equal to