1/6×9＋1/9×12＋1/12×15＋1/15×18＋1/18×21＋1/21×24
=1/3(1/6-1/9)+1/3(1/9-1/12)+1/3(1/12-1/15)+1/3(1/15-1/18)
+1/3(1/18-1/21)+1/3(1/21-1/24)
=1/3(1/6-1/9+1/9-……+1/21-1/24)
=1/3(1/6-1/24)
=1/24