3 . Consider the following four sequences , where only one of them is arithmetic and only one of them is geometric :
a � 10 , 9 , 7 , 4 , n b � 10,12.5,15.625,19.53125 , n c � 10 , 8.9 , 7.8 , 6.7 , n d � 10,12,14,18 , n
( a ) State which sequence is
( i ) arithmetic ;
( ii ) geometric .
( b ) For the geometric sequence selected in ( a )( ii ),
[ 2 ]
( i ) write down the common ratio ;
( ii ) write down the fifth term , giving the answer in fraction ;
( iii ) find the sum of the first eight terms . [ 4 ]
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