若等差数列And的前n项和为Sn,S10=10,S30=120,则S20=? S10,(S20-S10),(S30-S20)也为等差数列,即 10,(S20-10),(120-S20)等差 10+(120-S20)=2×(S20-10) 130-S20=2×S20-20 3×S20=150 ∴S20=50
