原长：a cm第1次 用去 ：（0.5a-1）cm第2次 用去：[0.5×（a-0.5a+1）+1]cm∴还剩：a-（0.5a-1）-[0.5×（a-0.5a+1）+1]=a-0.5a+1-[0.5×0.5a+0.5+1]=a-0.5a+1-0.25a-1.5=0.25a-0.5（cm）...