1)The sum of the 4th term and the 5th term of an aritmetic sequence is 18 . If the sum of the first two terms is 42 , find the general term
2)There are 14 seats in the first row , After the first row , the number of sets in each row is two more than the row in front of it .
a)express the number of seats in the nth in terms of n
b- If the last row has 28 seats , how many seats are there ?
two Qs of arithmetic sequence
2007-09-15 5:00 pm
回答 (1)
2007-09-15 5:33 pm
✔ 最佳答案
1)let general term ( the nth term )= x + (n - 1)y(x+ 3y)+(x+4y) = 18 - - - - 1
x+(x+y) = 42- - - - - 2
@1 -@2
6y = - 24
y = -4
x = (42 - y)/2
=46/2
=23
general term ( the nth term )= 23 - 4(n - 1)
2a)the number of seats in the nth in terms of n = 14 + 2(n - 1)=12+2n
b)12+2n = 28
2n = 16
n = 8
8 rows of seat
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