公差为1/2的等差数列,S100=145,则a2+a4+a6+...+a100=?S‹100›=100a₁+[100×99×(1/2)]/2=100a₁+2475=145,故a₁=-23.3偶数项公差=2×(1/2)=1,项数为50,首项a₂=a₁+d=-23.3+0....