根据材料内容，选择正确答案。

    Mr Smith had a week's holiday, so he said, "I'm going to see the sea by train." He put on his best clothes, took a small bag, went to the railway station and got onto the train. He wore a beautiful hat, and he often put his head out of the window during the trip and looked at the beautiful scenery. But suddenly the wind pulled his hat off.

    Mr Smith took his old bag and threw it out of the window quickly too. The others in the train laughed. "Is your bag going to bring back your beautiful hat?" they asked.

    "N0," Mr Smith answered, "but there is no name or address in my hat, and there's a name and an address on the bag. If someone finds both of them, he or she will send me the bag and the hat.''

如图所示，一定质量的理想气体由a状态变化到b状态，则此过程中（   ）

①2a+c＜0；②若（﹣ $\frac{3}{2}$ ，y₁），（﹣ $\frac{1}{2}$ ，y₂），（ $\frac{1}{2}$ ，y₃）在抛物线上，则y₁＞y₂＞y₃；③关于x的方程ax²+bx+k=0有实数解，则k＞c﹣n；④当n=﹣ $\frac{1}{a}$ 时，△ABP为等腰直角三角形．

其中正确结论是____（填写序号）．