Question

what does C.O.Bs means​

Answer 1

COBS

stands for cubing on both sides

$a = x + y \\ cobs \\ {(a)}^{3} = {(x + y)}^{3} \\ {a}^{3 } = {x}^{3} + {y}^{3} + 3 {x}^{2} y + 3 x{y}^{2}$

hope it helps