利用(n+1)^4-n^4=4n^3+6n^2+4n+1。給n賦值可得自然數立方和。得(n+1)^4-1=4(1+2^3+3^3十…十n^3)十n(n+1)(2n+1)+2n(n+1)+n。化簡得自然數立方和為(n(n+1))^2/4