A3 + B3 + C3 – 3ABC = A3 + (B + C)3 – 3BC (B + C) – 3ABC= (A + B + C){A2 – A(B + C) + (B + C)2} – 3BC (A + B + C)= (A + B + C) (A2 – AB – AC + B2 + 2BC + C2 – 3BC)= (A + B + C) (A2 + B2 + C2 – AB – AC – BC)

1. - DE THI 8
Lớp 8 Toán DE THI 8
1. a

3

+ b

3

+ c

3

– 3abc = a

3

+ (b + c)

3

– 3bc (b + c) – 3abc= (a + b + c){a

2

– a(b + c) + (b + c)

2

} – 3bc (a + b + c)= (a + b + c) (a

2

– ab – ac + b

2

+ 2bc + c

2

– 3bc)= (a + b + c) (a

2

+ b

2

+ c

2

– ab – ac – bc)