∵an=n[cos(nπ)/3-sin(nπ)/3]=n ×cos(2π/3)n ∵n取1到n时,cos(2π/3)n的取值依次为-1/2,-1/2,1,-1/2,-1/2,1,…… ∴S10=-1/2×(1+2+4+5+7+8+10)+3+6+9 =-1/2×(1+2+3+4+5+6+7+8+9+10)+3/2×(3+6+9) =-1/2×10×11×21/6+3/2×154 =38.5