1, [ ( a + b ) - c ]² = ( a + b )² - 2 . ( a + b ) . c + c²
= a² + 2ab + b² - 2ac - 2bc + c²
2, ( a - b + c )² = [( a - b ) + c ]²
= ( a - b )² + 2 . ( a - b ) . c + c²
= a² - 2ab + b² + 2ac - 2bc + c²
3, ( a - b - c )² = [( a - b ) - c ]²
= ( a - b )² - 2 . ( a - b ) . c + c²
= a² - 2ab + b² - 2ac + 2bc + c²
4, ( a + b + c + d )² = [ ( a + b + c ) + d ]²
= ( a + b + c )² + 2 . ( a + b + c) . d + d²
= [ ( a + b ) + c ]² + 2ad + 2bd + 2cd + d²
= ( a + b )² + 2 . ( a + b ) . c + c² + 2ad + 2bd + 2cd + d²
= a² + 2ab + b² + 2ac + 2bc + c² + 2ad + 2bd + 2cd + d²