已知1x﹣1y=1x+y,则yx﹣xy﹣2=____
He exercises in a sports____(中心)every day.
沈括《梦溪笔谈》载:“世间锻铁所谓钢铁者,用柔铁屈盘之,乃以生铁陷其间,泥封炼之,锻令相入,谓之团钢。”这项技术是