已知：P是正方形ABCD对角线BD上一点，PE⊥DC，PF⊥BC，E、F分别为垂足．

求证：AP=EF．

    I have a pencil box. It's blue. There are lots of things in it. I have a pen. It's red and long. I like it. I have three pencils. They are short. I have a ruler. It's big. I have an eraser. It's small.

拖拉机厂原计划全年生产拖拉机1320台，实际仅用10个月就完成了全年计划，照这样计算，全年可超产____台？

设（5a+3b）²=（5a﹣3b）²+A，则A=（　　）