∵等差数列且a4+a17=8∴a1+3d+a1+16d=8∴a1=-3/4a20=a1+（20-1）×d=35/4S偶=a2+a4+a6+……+a20S奇=a1+a3+a5+……+a19=S偶-10d=S偶-5∴S20=20×（-3/4+35/4）/2=80∵S20=S偶+S奇=80∴S偶+S偶-5=80∴S偶=85/2即a2+a4+...