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=?](/uploads/image/z/1819526-14-6.jpg?t=%E5%B7%B2%E7%9F%A5%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%2CS20%3D3S10%2CS30%3D36%2C%E5%88%99S10%3D%3F)
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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