等差数列的习题问题在等差数列{An}中,已知d=7,An=18,Sn=20,求A与n
来源:学生作业帮助网 编辑:作业帮 时间:2024/07/12 05:52:18
![等差数列的习题问题在等差数列{An}中,已知d=7,An=18,Sn=20,求A与n](/uploads/image/z/693482-50-2.jpg?t=%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%E7%9A%84%E4%B9%A0%E9%A2%98%E9%97%AE%E9%A2%98%E5%9C%A8%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%EF%BD%9BAn%7D%E4%B8%AD%2C%E5%B7%B2%E7%9F%A5d%3D7%2CAn%3D18%2CSn%3D20%2C%E6%B1%82A%E4%B8%8En)
x){uϦnx1';\4u@Ȳv̫}c훞_bkgkhgkdlc}y6IE˓k~
]
RhkbF:@RhbP1OS4 1y&X&y^q-D$Y4}{:^t>~O'${7%&$ف kp
等差数列的习题问题在等差数列{An}中,已知d=7,An=18,Sn=20,求A与n
等差数列的习题问题
在等差数列{An}中,已知d=7,An=18,Sn=20,求A与n
等差数列的习题问题在等差数列{An}中,已知d=7,An=18,Sn=20,求A与n
等差数列,An=A1+(n-1)d=A1+7n-7=18
A1+7n=25,A1=25-7n
Sn=n(A1+An)/2=n(A1+18)/2=20
A1n+18n=40
(25-7n)n+18n=40
7n^2-43n+40=0
n=5或8/7(舍去)
即n=5
A1=-10