Apps. and Interpretation for IBDP Maths Ebook 1 | Page 73

20
Paper 1 –Finding and with given terms
Example
The first three terms of a geometric sequence are 54, 18 and 6.
(a) Write down the value of r , the common ratio of the geometric sequence.
[1]
(b) Find u
7
.
[2]
(c) Find the sum of the first 10 terms of this sequence, giving the answer correct to
four decimal places.
[3]
Solution
(a)
(b)
1
r � A1 N1
3
u
�u � r �
7 1
7 1
[1]
8
(c)
7�1
�1
�
u7
�54� � �
�3
�
2
u7
�
27
A1 N2
S
S
10
10
u
�
1�
r
10
1(1 � r )
� �1
�
54 1�
� � �
�3
�
�
�
1
1�
3
10
�
�
�
(A1) for substitution
(M1) for valid approach
(A1) for substitution
[2]
S10 � 80.99862826
S10 � 80.9986
A1 N3
[3]
www.seprodstore.com
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