The 40th term of an arithmetic sequences is equal to the sum of the 20th and the 21st term. If the common difference is -10, find
a) the first term
b) the 10th term
help!!!Maths(AS)
2010-03-16 6:22 am
回答 (2)
2010-03-16 6:40 am
✔ 最佳答案
The 40th term of an arithmetic sequences is
equal to the sum of the 20th and the 21st
term. If the common difference is －10，find
a) the first term
Sol
Set a(1)=a
a(40)=a+39d
a(20)=a+19d
a(21)=a+20d
a+39d= 2a +39d
a=0
b) the 10th term
a(10)=a+9d=0+9*(－10)=－90
2010-03-16 6:54 am
T40 = T20 + T21 , d = -10 , find a & T10
a) a + (40 - 1)* -10 = [a + (20 - 1)* -10] + [a + (21 - 1)* -10]
a - 390 = (a - 190) + (a - 200)
a - 390 = 2a - 390
a = 0
b) T10 = 0 + (10 - 1)* -10
= -90
a) a + (40 - 1)* -10 = [a + (20 - 1)* -10] + [a + (21 - 1)* -10]
a - 390 = (a - 190) + (a - 200)
a - 390 = 2a - 390
a = 0
b) T10 = 0 + (10 - 1)* -10
= -90
參考: ME
收錄日期: 2021-04-24 23:20:19
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