1/42＋2又1/56＋3又1/72＋4又1/90
＝﹙4/168＋2又3/168 ﹚＋﹙3又5/360＋4又4/360﹚
＝2又1/24＋7又1/40
＝2又5/120＋7又3/120
＝9又1/15,