T500 =（S1+S2+……+S500）/500 =[500xa1+（500-1）a2+……+a500]/500 =2004 500xa1+（500-1）a2+……+a500=2004x500=4x500x501 T501 =[12x501+（501-1）xa1+……+a500]/501 =[12x501+4x500x501]/501 =2012
