3a8=5a133(a1+7d)=5(a1+12d)3a1+21d=5a1+60d2a1+39d=0因为a1＞0所以d＜0所以a20=a1+19d=(2a1+38d)/2=(2a1+39d-d)/2=-d/2＞0a21=a1+20d=(2a1+40d)/2=(2a1+39d+d)/2=d/2＜0所以Sn取最大值时n=20