Apps. and Interpretation for IBDP Maths Ebook 1 | Page 69

(c) un
� 888
12n �5 � 888
(M1) for setting inequality
12n � 893
893
n � (A1) for correct value
12
Thus, the smallest value of n is 75. A1 N3
n
� � � (M1) for valid approach
2
n
� (12n�
2)
2
(d) S �2(7) ( n 1)(12) �
S
n
n
2
Sn
6n n
� � A1 N2
[3]
[2]
Exercise 19
1. In an arithmetic sequence, u4 � 43 and S80 �� 5320 . Let u
1
and d be the first term and
the common difference of the arithmetic sequence respectively.
7
(a) (i) Write down the first equation connecting u
1
and d .
(ii) Write down the second equation connecting u
1
and d .
(b)
(iii) Hence, write down the values of u
1
and d .
Find u
n
in terms of n .
(c) Find the greatest value of n such that un
� 0.
(d)
Find S
n
in terms of n .
(e) Find the value of n such that Sn
�� 4425 .
[6]
[2]
[3]
[2]
[3]
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