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Paper 1 – Sum to Infinity
Example
The first three terms of a geometric sequence are u1 � 800, u2 � 720 and u3 � 648 .
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( a ) Find the value of r .
( b ) Find the value of S
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.
( c ) Find the sum to infinity of this sequence .
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Solution
( a )
( b )
( c )
|
720 r �
800
|
( M1 ) for valid approach |
|
r �
0.9
|
A1 |
S
6 u
� 1� r
6
1( 1 � r )
6
800 ( 1 � ( 0.9 ) )
S6
�
1�
0.9 S6 � 3748.472 S6 � 3750 A1
( A1 ) for substitution
|
u1 S
� 1� r
|
|
800
S �
� 1 �
0.9
|
( A1 ) for substitution |
|
S � �
8000
|
A1 |
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