已知等差数列前n项和为Sn,S20=3S10,S30=36,则S10=?

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已知等差数列前n项和为Sn,S20=3S10,S30=36,则S10=?已知等差数列前n项和为Sn,S20=3S10,S30=36,则S10=?已知等差数列前n项和为Sn,S20=3S10,S30=3

已知等差数列前n项和为Sn,S20=3S10,S30=36,则S10=?
已知等差数列前n项和为Sn,S20=3S10,S30=36,则S10=?

已知等差数列前n项和为Sn,S20=3S10,S30=36,则S10=?
已知等差数列前n项和为Sn,
S20=3S10,
a20+,+a11+S10=3S10;
a20+,+a11=2S10=2(a1+...+a10);
∴10d=2;
d=1/5;
S30=36,
S30=a30+...+a21+S20
=20d(a10+...+a1)+3S10;
=4S10+3S10
=7S10;
=36;
则S10=36/7;
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