a3-b3=（a-b）(a2+ab+b2)a3+b3=(a+b)(a2-ab+b2)
a³-b³=（a-b）(a²+ab+b²)a³+b³=(a+b)(a²-ab+b²) （X+1）³-（X-1）³=(x+1-x+1)[(x+1)²-(x+1)(x-1)+(x-1)²]=2[(x²+2x+1)-(x²-1)+(x²-2x+1)]=2[x²+2x+1-x²+1+x²-2x+1]=2[x²+3]=2x²+6
