题目

       Americans love sports--they love to play them, to watch them on television, and to talk about them. But this hobby sometimes has serious results-- at least to the players. For example, when people play tennis, sometimes they hurt their elbows(肘部) and in this way they develop tennis elbow. Also it is easy to hurt a knee in a football game. These injuries(损伤)happen while the player is having fun, but they still hurt.      A few months ago Kathleen Simmons, who loves playing volleyball, hurt her knee in a volleyball game. Her doctor told her that she needed a very difficul计算45×（270+6 5÷5）时，第一步应算除除法，第二步应算加加法，第三步算乘乘法．0不能作除数除数．