1/2＋5/6－7/12＋9/20－11/30＋13/42－1/7
=1-1/2+1/2+1/3-（1/3+1/4）+1/4+1/5-（1/5+1/6）+1/6+1/7-1/7
=1-1/2+1/2+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7-1/7
=1