9 . The first two terms of an infinite geometric sequence are
4
4
20cos � respectively , where � �� � � . 3
2
30cos � and
( a ) Find the common ratio r , giving the answer in terms of � . [ 2 ]
( b ) Hence , find the range of values of r . [ 2 ]
90 ( c ) Show that S � � 2
3tan � � 1 . [ 4 ]
4 ( d ) Hence , show that S � attains its minimum at � � � . 3
[ 5 ]
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