设n为给定的不小于3的正整数，数集P={x|x≤n，x∈N^*}，记数集P的所有k（1≤k≤n，k∈N^*）元子集的所有元素的和为P_k．

（1）求P₁，P₂；

（2）求P₁+P₂+…+P_n．

— I'd love to, but I think we shouldn't __________ our progress. We also have a long way to go.

I saw the doctor_____ in the city of Shanghai last year. Maybe he comes from Shanghai.  

某溶液中可能含有SiO₃^2﹣、OH^﹣、CO₃^2﹣、SO₄^2﹣、AlO₂^﹣、HCO₃^﹣、Na⁺、Mg²⁺、Fe³⁺、Al³⁺等离子．当向该溶液中逐滴加入一定物质的量浓度的盐酸溶液时，发现生成沉淀的物质的量随盐酸溶液的体积变化如图所示．下列说法正确的是（　　）