等差数列an的前n项和为Sn,且6Sn-5Sn=5,则a4=?答案给了个设首项公差为a1 d 原式得6(a1+3d)=2.所以a4=1/3
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![等差数列an的前n项和为Sn,且6Sn-5Sn=5,则a4=?答案给了个设首项公差为a1 d 原式得6(a1+3d)=2.所以a4=1/3](/uploads/image/z/5929164-36-4.jpg?t=%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97an%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%2C%E4%B8%946Sn-5Sn%3D5%2C%E5%88%99a4%3D%3F%E7%AD%94%E6%A1%88%E7%BB%99%E4%BA%86%E4%B8%AA%E8%AE%BE%E9%A6%96%E9%A1%B9%E5%85%AC%E5%B7%AE%E4%B8%BAa1+d+%E5%8E%9F%E5%BC%8F%E5%BE%976%EF%BC%88a1%2B3d%EF%BC%89%3D2.%E6%89%80%E4%BB%A5a4%3D1%2F3)
等差数列an的前n项和为Sn,且6Sn-5Sn=5,则a4=?答案给了个设首项公差为a1 d 原式得6(a1+3d)=2.所以a4=1/3
等差数列an的前n项和为Sn,且6Sn-5Sn=5,则a4=?
答案给了个设首项公差为a1 d 原式得6(a1+3d)=2.所以a4=1/3
等差数列an的前n项和为Sn,且6Sn-5Sn=5,则a4=?答案给了个设首项公差为a1 d 原式得6(a1+3d)=2.所以a4=1/3
S5+5(S5-S3)=5 S5+5(a4+a5)=5 5(2a4-2d)/2+5(2a4+d)=5 5a4-5d+10a4+5d=5 15a4=5 a4=1/3
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