\(\left(a-b\right)+\left(c-d\right)-\left(a+c\right)\\ =a-b+c-d-a-c\\ =-b-d\\ =-\left(b+d\right)\)
\(\left(a-b\right)-\left(c-d\right)+\left(b+c\right)\\ =a-b-c+d+b+c\\ =a+d\)
\(a\left(b-c\right)-a\left(b+d\right)\\ =a\left[b-c-\left(b+d\right)\right]\\ =a\left(b-c-b-d\right)\\ =a\left(-c-d\right)\\ =-a\left(c+d\right)\)
\(\left(a+b\right)\left(c+d\right)-\left(a+d\right)\left(b+c\right)\\ =ac+bc+ad+bd-\left(ab+ac+db+dc\right)\\ =ac+bc+ad+bd-ab-ac-db-dc\\ =bc+ad-ab-cd\\ =bc-cd+ad-ab\\ =c\left(b-d\right)-a\left(b-d\right)\\ =\left(b-d\right)\left(c-a\right)\)
pn khai triển vế trái của từng bài rồi rút gọn là ra thôi
