Question:
Let x, y, z be three positive real numbers in a geometric progression such that x < y < z. If 5x, 16y, and 12z are in an arithmetic progression then the common ratio of the geometric progression is
- 3/6
- 3/2
- 5/2
- 1/6
Correct Answer: 3
Since x, y ,and z are in G.P. and x<y<z, let x = a, y=ar and z=ar2, where a>0 and r>1.
It is also given that, 15x, 16y and 12z are in A.P.
Therefore, 2×16y=5x+12z
Substituting the values of x, y and z we get,
$32ar=5a + 12ar^2$
$\Rightarrow 32 r=5+12 r^{2}$
$\Rightarrow 12 r^{2}-32 r+5=0$
On solving the above quadratic equation we get r=1/6 or 5/2.
Since r>1, therefore r=5/2.
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