等差数列等差公式：an=a1+(n-1)d等差求和：Sn=n (a1+an)／2 =na1+n(n-1)d／2⑴公差为d的等差数列,各项同加一数所得数列仍是等差数列,其公差仍为d． ⑵公差为d的等差数列,各项同乘以常数k所得数列仍是等...