Xiao Ming used a semicircular piece of paper with a radius of 5 cm to make a straight cone paper hat in his school mathematics extracurricular activities. Find the length of the radius of the circle at the bottom of the paper cap.
小明在学校数学课外活动中，用一张半径5厘米的半圆形纸片，做成一个直圆锥纸帽．求纸帽底面圆的半径的长．
<result>2.5</result>