It was Sunday yesterday. The weather was fine. It was sunny and there were no clouds in the sky. Sandy and Sue got up early in the morning. Now they were in the park. It is a big park and it's so beautiful. They saw many people there. Some boys played football on the grass. Some girls danced under the tree. An old man walked beside the lake. A young woman and her little son looked at the fish in the lake. How happy they are!

分解因式：x²﹣4=____．

Fe²⁺ $\underset{④}{\overset{①}{\rightleftharpoons }}$  Fe³⁺ $\underset{③}{\overset{②}{\rightleftharpoons }}$  FeO₄^2﹣（高铁酸根离子）

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