S10=5(a1+a10)=100 a1+a10=20 S100=50(a1+a100)=10 a1+a100=0.2 a100-a10=90d=-19.8 d=-0.22 a1+a110=a1+a100+10d=0.2+10*(-0.22)=-2 S110=55(a1+a110)=-2*55=-110
