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已知 \({a^{\tfrac{1}{2}}} + {a^{ - \tfrac{1}{2}}} = 3\),求下列各式的值.
②\({a^2} + {a^{ - 2}}\);
参考答案:②\(\because {(a + {a^{ - 1}})^2} = {a^2} + {a^{ - 2}} + 2 = 49\)\(\therefore {a^2} + {a^{ - 2}} = 47\)
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