Analysis and Approaches for IBDP Maths Ebook 1 | Page 66

Your Practice Set – Analysis and Approaches for IBDP Mathematics
21
Paper 1 Section A – Finding
and
with given first three terms
Example
The first three terms of an infinite geometric sequence are 54, 18 and 6.
(a)
Write down the value of r , the common ratio of the geometric sequence.
(b) Find u
7
.
(c)
Find the sum to infinity of this sequence.
[1]
[2]
[2]
Solution
(a)
(b)
1
r � A1 N1
3
u
�u � r �
7 1
7 1
7�1
�1
�
u7
�54� � �
�3
�
2
u7
�
27
A1 N2
(A1) for substitution
u1
(c) S � �
1� r
54
S �
�
A1
1
1�
3
S �
� 81
A1 N1
[1]
[2]
[2]
58
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