等差数列和=（首项+末项）*项数/2 S =(a1+an)*n/2
等差数列和= 项数*首项 + 项数*（项数-1）*公差/2 S=n*a1+n*(n-1)*d/2
末项＝首项+（项数-1）*公差 an=a1+(n-1)*d
项数＝（末项－首项）/公差+1 n=(an-a1)/d+1
首项＝末项－（项数-1）*公差 a1=an-(n-1)*d