Gọi dd là công sai của cấp số đã cho
Ta có: S100=50(2u1+99d)=24850⇒d=497−2u199=5S100=50(2u1+99d)=24850⇒d=497−2u199=5
⇒5S=5u1u2+5u2u3+...+5u49u50⇒5S=5u1u2+5u2u3+...+5u49u50
=u2−u1u1u2+u3−u2u2u3+...+u50−u49u49u50=u2−u1u1u2+u3−u2u2u3+...+u50−u49u49u50
=1u1−1u2+1u2−1u3+...+1u48−1u49+1u49−1u50=1u1−1u2+1u2−1u3+...+1u48−1u49+1u49−1u50
=1u1−1u50=1u1−1u1+49d=245246=1u1−1u50=1u1−1u1+49d=245/246
⇒S=49/246⇒S=49/246.