Apps. and Interpretation for IBDP Maths Ebook 1 | Page 289

(b)
(iii)
(i)
P( X �11� X � 9)
P( X �11 X � 9) �
P( X � 9)
P( X �11)
P( X �11 X � 9) �
P( X � 9)
(M1) for valid approach
2
P( X �11 X � 9) � 36
(A1) for substitution
1 1
�
12 12
1
P( X �11 X � 9) � A1 N3
3
5
6
A1
N1
[7]
(ii) E( X ) � 0
(2)P( X �10) � (1)P( X �10)
�( �k)P( X �10) � 0
(M1)(A1) for correct formula
� 1 � � 1 � � 5 �
(2) � � � (1) � � � ( �k) � � � 0
�12 � �12 � � 6 �
(A1) for substitution
2�1�10k
� 0
3
k �
10
A1 N4
[5]
19
Exercise 81
1. In a game, two standard six-sided dice are tossed. Let X be the sum of the scores on the
two dice.
(a)
Find
(i) P( X � 5) ;
(ii) P( X � 5) ;
(iii) P( X �4 X � 6) .
[7]
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