Respuesta :

gmany

If a, b, c is a geometric sequence, then [tex]ac=b^2[/tex]

We have:

[tex]a_{11}=72;\ a_{12};\ a_{13}=\dfrac{1}{2}[/tex]

substitute:

[tex](a_{12})^2=72\cdot\dfrac{1}{2}\\\\(a_{12})^2=36\to a_{12}=\pm\sqrt{36}\\\\a_{12}=-6\ \vee\ a_{12}=6[/tex]

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