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