SOLUTION: If three geometric means are inserted between 120 and 15/2,find the third of these geometric means

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Question 1044745: If three geometric means are inserted between 120 and 15/2,find the third of these geometric means
Answer by KMST(5328) About Me  (Show Source):
You can put this solution on YOUR website!
If 3 geometric means are inserted between 120 and 15%2F2 , those five numbers form a geometric sequence.
Also, all five terms are positive, meaning that so is the common ratio.
If , b%5B2%5D=120%2Ar , b%5B3%5D=120%2Ar%5E2 , b%5B4%5D=120%2Ar%5E3
, and b%5B5%5D=120%2Ar%5E4=15%2F2 .
So,
120%2Ar%5E4=15%2F2<-->r%5E4=%2815%2F2%29%281%2F120%29<-->r%5E4=1%2F16<-->r=1%2F2
The thirds of those geometric means can be calculated as
b%5B4%5D=120%2A%281%2F2%29%5E3--->b%5B4%5D=120%2F8--->b%5B4%5D=highlight%2815%29 ,
or more easily as b%5B4%5D=b%5B5%5D%281%2Fr%29=%2815%2F2%29%2A2=15 .

Alternatively, we can calculate the geometric mean of 120 and 15%2F2 ,
which would be the second of the three means to be inserted, as
sqrt%28120%2A%2815%2F2%29%29=sqrt%2860%2A15%29=sqrt%2815%2A4%2A15%29=15%2A2=30 .
Then we can calculate the third of the three means to be inserted as the geometric mean of 30 and 15%2F2 ;
sqrt%2830%2A%2815%2F2%29%29=sqrt%2815%2A15%29=15 .