Nếu \({1 \over 2}\left( {{a...

D

- Hướng dẫn giải

Ta có:

\(\dfrac{1}{2}\left( {{a^\alpha } + {a^{ - \alpha }}} \right) = 1 \)

\(\Leftrightarrow {a^\alpha } + {a^{ - \alpha }} = 2 \)

\(\Leftrightarrow {a^\alpha } + \dfrac{1}{{{a^\alpha }}} = 2\)

\(\Leftrightarrow {\left( {{a^\alpha }} \right)^2} - 2{a^\alpha } + 1 = 0\)

\(\Leftrightarrow {a^\alpha } = 1 \Leftrightarrow \alpha = 0\)