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