Ta có : A + B
= ( a + b - 5 ) + ( -b - c - 1 )
= a + b - 5 + (-b) - c - 1
= a + ( -c ) + ( - b + b ) + ( -5 + 1)
= a + ( -c ) + 0 + ( -4 )
Suy ra: A + B = a + ( -c ) + 0 + ( -4 )
ta có : C - D
= ( b - c - 4 ) - ( a + b )
= b - c - 4 + ( -a ) - b
= a + ( -c ) + ( -b + b ) + ( -4 )
= a + ( -c ) + 0 + ( -4 )
= a + ( -c ) + ( -4 )
Suy ra : C - D = a + ( -c ) + ( -4 )
➡️ A + B = a + ( -c ) + ( -4 ) = C - D