$请问：\displaystyle \frac{1}{2} + \displaystyle \frac{1}{6} + \displaystyle \frac{1}{12} + \displaystyle \frac{1}{20} + \displaystyle \frac{1}{30} + \displaystyle \frac{1}{42} + \displaystyle \frac{1}{56} + \displaystyle \frac{1}{72} + \displaystyle \frac{1}{90} = ?$