证明这个公式成立这一点就留作为读者 的一个习题。 It is left as a problem to the reader to show that this formula holds 解这个方程花了我们两个多小时。 It took us more than two hours to solve this equaton. 做这些练习对你们是有好处的证明这个公式成立这一点就留作为读者 的一个习题。 It is left as a problem to the reader to show that this formula holds. 解这个方程花了我们两个多小时。 It took us more than two hours to solve this equation. 做这些练习对你们是有好处的