HKDSE Maths

2012-04-08 9:17 am
The sume of the 4th term and the 5th term of a geometric sequence is -4. If the sum of the first two terms is 32, find the first term of the sequence.

回答 (2)

2012-04-08 7:57 pm
✔ 最佳答案
4th + 5th
= ar³ + ar ⁴= - 4 ...... (1)1st + 2nd
= a + ar = 32 ...... (2)
(1) / (2) :( ar³ + ar ⁴) / (a + ar) = - 4 / 32
ar³ (1 + r) / ( a (1 + r) ) = - 1/8
r³ = - 1/8
r = - 1/2
Sub. into (2) :a + a(- 1/2) = 32
a/2 = 32
a = 64
The first term of the sequence is 64.
2012-04-09 11:07 am
如果是應付選擇題(multiple choice),以下是最快的方法:
公式: y / ( 1 + ( x/y ) ^ ( 1/d ) )
= 32 / ( 1 + (-4/32) ^ (1/3) )
= 64

熟練的同學應該可以十秒內準確地完成題目。

詳情:

先將題目變為:
The sum of the (d + 1) th term and the (d + 2) th term of a geometric sequence is x.
If the sum of the first two terms is y.

在此題目,
(d+1)=4
(d+2) = 5
d=3
x = -4
y = 32

The first term of the sequence
= y / ( 1 + ( x/y ) ^ ( 1/d ) )
= 32 / ( 1 + (-4/32) ^ (1/3) )
= 32 / ( 1 + (-1/8) ^ (1/3) )
= 32 / ( 1 + (-1/2) )
= 32 / ( 1 - 1/2)
= 32 / ( 1/2 )
= 32 / ( 0.5 )
= 64

註:
(-1/8) ^ (1/3) 的意思是
負八份之一的開方三次
即係負二份之一(i.e. -1/2)

同學可以到以下網址練習此類題目:
http://www.ktlam.hk/hkmath/index/d3dc2f17


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