Overview: 1 questions will be solved this time.Among them
           ☆1 inequalities

[ 1/1Inequality]
    Assignment:Find the solution set of inequality (a-9)(a+1) <0 .
    Question type: Inequality
    Solution:
    The inequality can be reduced to 1 inequality：
         ( a - 9 ) * ( a + 1 ) <0         (1)

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    From inequality(1)：
         -1 < a < 9

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    The final solution set is :
         -1 < a < 9

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