(a+b)^3 =(a+b)(a+b)(a+b) = (a^2+2ab+b^2)(a+b)=a^3 + 3(a^2)b + 3a(b^2)+ b^3
　　a=n；b=5m²
（n-5m²）³ =　n³-15n²m²+75nm^4-125m^6