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At present it is very difficult to predict all the RNA secondary structures. This paper describes RNA secondary structure prediction methods from 1970\' to this day in a systematic way some. Tracing back and following the tracks of prediction methods the author made a summary of RNA research progress and situation while a enumeration problem of RNA secondary structure is studied with the method of combinatorial enumeration.

目前所有有关RNA二级结构的预测算法要预测出所有序列的二级结构仍然很困难，本文系统介绍了从上世纪70年代以来发展至今的各类RNA二级结构预测算法，从对RNA结构预测方法的追溯和跟踪，对当今RNA研究的进展和形势做了一个总结，并且应用组合计数的技巧解决了一类RNA二级结构的计数问题。

Listening comprehension is the key to the command of any foreign language. That's one of the reasons why it's so difficult to learn on your own.

听力是掌握任何一门外语的关键，这也是它为什么自学起来如此困难之原因所在。

It is hard and tough in texture and difficult to comminute so that active constituents is uneasy to be extracted, thus prosessed products are often used.

因此，本课题对骨碎补炮制工艺及质量标准进行了研究，为临床用药更合理、更有效提供科学的理论依据。

It is difficult to find a comprehensive but simple source code to handle client/server communications.

找到一份全面但简单的解决客户端/服务器通信的源码是件困难的事情。

Poor in communization , difficult to maintain and enhance the fund , harmful to equal competition of labor force and urbanization .

但个人账户制也存在一些问题：共济性差，基金的保值增值困难，不利于劳动力市场的平等竞争及农民工的城镇化。

Due to the constraint of both high current and high speed, it is difficult to commutate for high-speed and high-current DC machine .

1前言 换向问题是直流电机的固有问题，换向过程十分复杂，影响换向的因数很多。

Magnetic drive pump mechanical seal is also very difficult to competently adapt to the high-vacuum conditions of use.

技术上的这些突破和其它改进措施，使磁力驱动泵的可靠性和经济性有了惊人的提高。

Since it is difficult to achieve complete orthogonal spread spectrum code sequence, and sometimes impossible to achieve orthogonal in the CDMA system, there were major multiple-access interference, greatly limiting the capacity of the system users.

摘要由于扩频码序列很难实现完全正交，有时甚至是不可能达到正交，在CDMA系统中存在着严重的多址干扰，极大地限制了系统用户容量。

It is difficult to implement the face recognition mechanism using computers for several reasons. Human face is a deformable object composing of complex 3D curve surfaces, which is hard to be represented in form of mathematics.

计算机人脸识别是一个复杂而困难的问题，其原因是：人脸是由复杂的三维曲面构成的可变形体，难以用数学描述；再者，所有人的人脸结构高度相似，而人脸的图像又受年龄和成像条件的影响，这使得同一人在不同条件下的差别可能比不同的人在相同条件下的差别还要大。

It is difficult to make due to the complex technology and large quantity of special module...

另外 ，技术复杂，专用组件多，制造难度大。

Breath, muscle contraction of the buttocks; arch body, as far as possible to hold his head, right leg straight towards the ceiling (peg-leg knee in order to avoid muscle tension).

呼气，收缩臀部肌肉；拱起身体，尽量抬起头来，右腿伸直朝向天花板（膝微屈，以避免肌肉紧张）。

The cost of moving grain food products was unchanged from May, but year over year are up 8%.

粮食产品的运输费用与5月份相比没有变化，但却比去年同期高8％。