因为4/21=1/3-1/7
16/63=1/7+1/9
1/42=1/6-1/7
所以原式=
5/6+6/7+8/9+(1/3-1/7)+(1/7+1/9)+(1/6-1/7)
=(5/6+1/6)+(8/9+1/9)+1/3+(6/7-1/7)
=1+1+1/3+5/7
=64/21