F.5_______Maths
2007-11-06 2:39 am
The sum of the first five terms of an arithmetic sequence is 15. If the fourth term is 7, the first term is...?
回答 (1)
2007-11-06 3:18 am
✔ 最佳答案
Let T(n)=a+ndT(1)=a+d
T(2)=a+2d
T(3)=a+3d
T(4)=a+4d
T(5)=a+5d
T(1)+T(2)+T(3)+T(4)+T(5)=5a+15d=15 ---(1)
T(4)=a+4d=7 --(2)
(2)x5 - (1):
5d=20 => d=4 ---(3)
Sub (3) into (2):
a=7-4x4=-9
Therefore, T(n)=-9+4n
First term T(1)= a+d = -9 + 4 = -5
參考: me
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