Analysis and Approaches for IBDP Maths Ebook 1 | Page 60

Your Practice Set – Analysis and Approaches for IBDP Mathematics
18
Paper 1 Section A – Finding the number of terms
Example
Consider the arithmetic sequence 13, 19, 25, 31, ….
(a) Find u
92
.
(b) Find the value of n so that un
� 385 .
[3]
[3]
Solution
(a) d � 6
(A1) for correct d
u92 �13 � (92 � 1)(6)
(M1) for using u
n
u92 � 559
A1 N3
(b) 385 �13 � ( n � 1)(6)
(M1) for correct approach
372 �6( n � 1)
(A1) for simplification
n � 63
A1 N2
[3]
[3]
Exercise 18
1. Consider the arithmetic sequence 50, 47, 44, 41, ….
(a) Find u
83
.
(b) Find the value of n so that un
�� 319 .
[3]
[3]
2. Consider the arithmetic sequence 17.5, 18, 18.5, 19, ….
(a)
Find u
n .
(b) Find the value of n so that un
� 40.
[3]
[3]
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