1×3分之1+3×5×分之1+5×7分之1+7×9分之1+9×11分之1+11×13分之1+13×15分之1
=(1/2)×(1-1/3)+(1/2)×(1/3-1/5)+(1/2)×(1/5-1/7)+(1/2)×(1/7-1/9)+(1/2)×(1/9-1/11)+(1/2)×(1/11-1/13)+(1/2)×(1/13-1/15)
=(1/2)×(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13+1/13-1/15)
=(1/2)×(1-1/15)
=1/2×14/15
=7/15