- #1
Titan97
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Homework Statement
If three number are chosen randomly from the set ##{1,3,3^2,...3^n}## without replacement, then the probability that they form an increasing geometric progression is?
(a) 3/2n if n is odd
(b) 3/2n if n is even
(c)3n/2(n² -1) if n is even
(d) 3n/2(n² -1) if n is odd
Homework Equations
None
The Attempt at a Solution
This problem can be easily solved if I chose the end terms first.
Let ##3^a## be the first term and ##3^b## be the third term. Then the middle term has to be ##3^{\frac{a+b}{2}}##.
Now, both ##a## and ##b## have to be even or both has to be odd.
let ##n## be odd. Then number of terms with even exponents=##\frac{n+1}{2}##
number of terms with odd exponents=##\frac{n+1}{2}##
Total number of ways of choosing 2 numbers ##3^a## and ##3^b## = ##2\cdot ^{\frac{n+1}{2}}C_2##
Probability is $$P(n)=\frac{2\cdot ^{\frac{n+1}{2}}C_2}{^{n+1}C_3}$$
But I am not getting the answers given in the options.