a1=2/3,an=-2/3,n=14,求相应等差数列{An}的前N项和Sn
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a1=2/3,an=-2/3,n=14,求相应等差数列{An}的前N项和Sn
![a1=2/3,an=-2/3,n=14,求相应等差数列{An}的前N项和Sn](/uploads/image/z/18705564-36-4.jpg?t=a1%3D2%2F3%2Can%3D-2%2F3%2Cn%3D14%2C%E6%B1%82%E7%9B%B8%E5%BA%94%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7BAn%7D%E7%9A%84%E5%89%8DN%E9%A1%B9%E5%92%8CSn)
∵是等差数列
∴d=(a14-a1)/13=-4/39
∴an=a1+(n-1)d=2/3-4(n-1)/39=(30-4n)/39
∴Sn=(a1+an)n/2=(43n-4n^2)/78
∴d=(a14-a1)/13=-4/39
∴an=a1+(n-1)d=2/3-4(n-1)/39=(30-4n)/39
∴Sn=(a1+an)n/2=(43n-4n^2)/78
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