[tex](a + b + c - d) ^{2} = \\ \\ (a + b + c - d)(a + b + c - d) = \\ \\ a(a + b + c - d) + b(a + b + c - d) + c(a + b + c - d) - d(a + b + c - d) = \\ \\ {a}^{2} + ab + ac - ad + ba + {b}^{2} + bc - bd + ca + cb + {c}^{2} - cd - da - db - dc + {d}^{2} = \\ \\ {a}^{2} + (ab + ab) + (ac + ac) + ( - ad - ad) + {b}^{2} + (bc + bc) + ( - bd - bd) + {c}^{2} + ( - cd - cd) + {d}^{2} = \\ \\ {a}^{2} + 2ab + 2ac - 2ad + {b}^{2} + 2bc - 2bd + {c}^{2} - 2cd + {d}^{2} [/tex]
