b2-22b1 b b2 2 (3) 21 c11a22-a21412 22 21a22 代入(1)得 a21a22 此处b△ ad-bc称为二阶行列式12 11 21 22 1 11 12 2 2 a aa a b aa b x −− = 11 22 21 12 11 2 21 1 a a a a a b a b −− = , 21 22 11 12 21 2 11 1 a a a a a b a b = (3) 代入(1) 得 , 21 22 11 12 2 22 1 12 1 a a a a b a b a x = (4) 此处 ad bc称为二阶行列式. c d a b = − 