Question

[Ngọc Anh Dũng]

Sử dụng tính chất phân phối giữa phép cộng và phép nhân để đưa các tích sau về tổng :

a) ( a + b ) . ( a - b )

b) ( a + b ) ³

c) ( a + b ) . ( a² - ab + b² )

d) ( a - b ) . ( a² + ab + b² )

e) ( a - b )³

Answer 1

[I don't need you]

a) \(\left(a+b\right).\left(a-b\right)=a.\left(a-b\right)+b.\left(a-b\right)=a^2-ab+ba-b^2\)\(=a^2-b^2\)

b) \(\left(a+b\right)^3=\left(a+b\right).\left(a+b\right).\left(a+b\right)=a.\left(a+b\right).\left(a+b\right)+b.\left(a+b\right).\left(a+b\right)\)

\(=\left(a^2+ab\right).\left(a+b\right)+\left(ba+b^2\right).\left(a+b\right)\)\(=a^2.\left(a+b\right)+ab.\left(a+b\right)+ba.\left(a+b\right)+b^2.\left(a+b\right)\)

\(=a^3+a^2b+a^2b+ab^2+ba^2+b^2a+b^2a+b^3\)\(=a^3+3a^2b+3ab^2+b^3\)

c) \(\left(a+b\right).\left(a^2-ab+b^2\right)=a.\left(a^2-ab+b^2\right)+b.\left(a^2-ab+b^2\right)\)

\(=a^3-a^2b+ab^2+ba^2-ab^2+b^3\)\(=a^3+b^3\)

d) \(\left(a-b\right).\left(a^2+ab+b^2\right)=a.\left(a^2+ab+b^2\right)-b.\left(a^2+ab+b^2\right)\)

\(=a^3+a^2b+ab^2-ba^2-ab^2-b^3\)\(=a^3-b^3\)

e) \(\left(a-b\right)^3=\left(a-b\right).\left(a-b\right).\left(a-b\right)=a.\left(a-b\right).\left(a-b\right)-b.\left(a-b\right).\left(a-b\right)\)

\(=\left(a^2-ab\right).\left(a-b\right)-\left(ba-b^2\right).\left(a-b\right)\)\(=a^2.\left(a-b\right)-ab.\left(a-b\right)-ba.\left(a-b\right)+b^2.\left(a-b\right)\)

\(=a^3-a^2b-a^2b+ab^2-ba^2+b^2a+b^2a-b^3\)

\(=a^3-3a^2b+3ab^2-b^3\)

Answer 2

[Nguyễn Hoàng Nam Thiên]

1) (a+b).(a+b)=(a+b)2=a2+2ab+b2

2) (a-b)2=a2-2ab+b2

3) (a+b).(a-b)=a2-b2

4) (a+b)3=a3+3a2b+3ab2+b3

5) (a-b)3=a3-3a2b+3ab2-b3

6) (a+b).(a2-ab+b2)=a3+b3

7) (a-b).(a2+ab+b2)=a3-b3