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下面是小李的证明: 指数和三角函数在域上的所有点都是可微和连续的, 而−2≤x≤−1-2\leq x\leq -1−2≤x≤−1minus, 2, is less than or equal to, x, is less than or equal to, minus, 1 在 ffff 的域内.
因此, 根据中值定理, f′(x)=14f'(x)=\dfrac14f′(x)=41f, prime, left parenthesis, x, right parenthesis, equals, start fraction, 1, divided by, 4, end fraction 在区间 −2<x<−1-2< x< -1−2<x<−1minus, 2, is less than, x, is less than, minus, 1 上一定有解.
