一节链条长度2.5cm，2节链条重合0.8cm,长度2.5+2.5-0.8=2.5+1.7=4.2cm，3节链条长4.2+2.5-0.8=4.2+1.7=5.9cm，n节链条长2.5+1.7（n-1)=0.8+1.7n cm ,4节链条长0.8+6.8=7.6 cm
