Puzzle...

Algebra Level 2

Cool puzzle!

Five numbers are in geometric progression. Their sum is 6 and the sum of their reciprocals is 3. What is their product?

4√2 8 16 2√4

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2 solutions

Let the numbers be a / r 2 , a / r , a , a r , a r 2 a/r^2, a/r,a,ar,ar^2

The sum is now given as a ( 1 / r 2 + 1 / r + 1 + r + r 2 ) = 6 a(1/r^2+1/r+1+r+r^2)=6

Sum of reciprocals is given by ( 1 / r 2 + 1 / r + 1 + r + r 2 ) / a = 3 (1/r^2+1/r+1+r+r^2)/a=3

Dividing the 2 equations, we get a 2 = 2 a^2=2

The product is given as a 5 a^5 which is 4 2 4\sqrt{2}

Christian Daang
Nov 23, 2014

Solution 1:

Let the numbers be a/r², a/r, a, ar, ar²

The reciprocals are r²/a, r/a,1/a, 1/ar, 1/ar²

Sum of the numbers = a/r² (r^5-1)/(r-1)

Sum of reciprocals = r²/a (1-1/r^5)/(1-1/r)

= r²/a [(r^5-1)/r^5]/[(r-1)/r] = (r^5-1)/(r-1)* r²/a * 1/r^4

= a/r² (r^5-1)/(r-1) * 1/a² = 6/a² = 3

=> a² = 2

Product = a^5 = 4√2

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