（1/2×3)＋(2/3×5)＋(3/5×8)＋(4/8×12)＋(5/12×17)＋(6/17×23)＋(7/23×30)
=1/2-1/3+1/3-1/5+1/5-1/8+1/8-1/12+1/12-1/17+1/17-1/23+1/23-1/30
=1/2-1/30
=7/15