SOLUTION: The fifth term of a geometric sequence is 15 and the eighth term is -15/8. Find the first term, common ratio and the third term.

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Question 730193: The fifth term of a geometric sequence is 15 and the eighth term is -15/8. Find the first term, common ratio and the third term.
Answer by mananth(16946) About Me  (Show Source):
You can put this solution on YOUR website!
t5 = a*r^4=15
t8 = ar^7= -15/8


t8%2Ft5+=+ar%5E7%2Far%5E4+=+-%2815%2F8%29%2F15
r%5E3=-1%2F8
r=-1/2
ar^4 = 15
a*(-1/2)^4=16
a*(1/16)=15
a= 240
T3 = 240*(1/4)
T3= 60
CHECK
t8= 240 * (-1/2)^7
= 240*(-1/128)
=-15/8