✔ 最佳答案
There is
a^3 + b^3 = (a+b)(a^2 - ab+b^2 )
a^3 - b^3= (a-b)(a^2 + ab +b^2 )
So...
(x+4)^3 - x^ 3 {apply the identity ---> a^3 - b^3 = (a-b)(a^2 + ab +b^2 ) }
= [(x+4) - x] [(x+4)^2 + x(x+4) + x^2]
= [x+4 - x] [(x^2 + 8x + 16) +(x^2 + 4x)+x^2 ]
= 4[ x^2 + 8x + 16+x^2 + 4x+x^2 ]
= 4[3x^2 +12x +16 ]
2006-11-07 22:00:13 補充:
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