2/2x4＋2/4x6＋2/6x8＋2/8x10＋2/10x12
=1/2-1/4＋1/4-1/6＋1/6-1/8＋1/8-1/10＋1/10-1/12
=1/2-1/12
=6/12-1/12
=5/12