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

[ 1/1Inequality]
    Assignment:Find the solution set of inequality 0 <= (40-2a)*(60-2a) <= 800 .
    Question type: Inequality
    Solution:
    The inequality can be reduced to 2 inequalities：
        0 <= ( 40 - 2 * a ) * ( 60 - 2 * a )         (1)
         ( 40 - 2 * a ) * ( 60 - 2 * a ) <= 800         (2)

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    From inequality(1)：
         a ≤ 20 或  a ≥ 30
    From inequality(2)：
         10 ≤ a ≤ 40

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    From inequalities (1) and (2)
         10 ≤ a ≤ 20 或  30 ≤ a ≤ 40    (3)

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    The final solution set is :
         10 ≤ a ≤ 20 或  30 ≤ a ≤ 40

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