某等差數列的第4項及第5項之和是18。若首兩項之和是42，求該數列的...
回答 (1)
2007-09-17 1:14 am
✔ 最佳答案
1 a )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)
b)the 15th term = 23 - 4( 15 - 1)
= - 33
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