PTĐTTNT: 8$a^{3}$+27$b^{3}$+64$c^{3}$-72abc nhanh mình vote 5 sao 04/09/2021 Bởi Alexandra PTĐTTNT: 8$a^{3}$+27$b^{3}$+64$c^{3}$-72abc nhanh mình vote 5 sao
8a³+ 27b³+ 64c³- 72abc = (2a)³+ (3b)³+ (4c)³- 72abc = (2a+ 3b)³+ (4c)³- 18ab(2a+3b)- 72abc = (2a+ 3b+ 4c)[(2a+3b)²- 4c(2a+3b)+ 16c²]- 18ab(2a+3b+4c) = (2a+ 3b+4c)(4a²+ 12ab+ 9b²- 8ac- 12bc+ 16c²- 18ab) = (2a+ 3b+4c)(4a²- 6ab+ 9b²- 8ac- 12bc+ 16c²) Bình luận
Đáp án:
Giải thích các bước giải:
8a³+ 27b³+ 64c³- 72abc
= (2a)³+ (3b)³+ (4c)³- 72abc
= (2a+ 3b)³+ (4c)³- 18ab(2a+3b)- 72abc
= (2a+ 3b+ 4c)[(2a+3b)²- 4c(2a+3b)+ 16c²]- 18ab(2a+3b+4c)
= (2a+ 3b+4c)(4a²+ 12ab+ 9b²- 8ac- 12bc+ 16c²- 18ab)
= (2a+ 3b+4c)(4a²- 6ab+ 9b²- 8ac- 12bc+ 16c²)