a等于负2b等于负1,求5ab的平方减大括号2a方b减中括号3ab的平方减小括号4ab的平方减2a方b

Question

a等于负2b等于负1,求5ab的平方减大括号2a方b减中括号3ab的平方减小括号4ab的平方减2a方b

Answer 1

5ab^2 - { 2a^2b - [ 3ab^2 - ( 4ab^2 - 2a^2b)]}
= 5ab^2 - { 2a^b - [ 2ab^2 - 4ab^2 + 2a^2b] }
= 5ab^2 - { 2a^2b + 2ab^2 - 2a^2b}
= 5ab^2 - 2ab^2
= 3ab^2
因为 a = 2b = - 1 所以有：a = - 1 b = -1/2
代入原式得：原式 = 3 × （- 1）× （-1/2)^2
= - 3/4