Kindly report to the management any inattention on the part of any employee.
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设A为m阶实对称矩阵且正定,B为m×n阶实矩阵,BT为B的转置矩阵,试证BTAB为正定矩阵的充分必要条件是矩阵B的秩r(B)=n.
It is, everyone agrees, a huge task that the child performs when he learns to speak, and the fact that he does so in so short a period of time challenges explanation. Language learning begins with listening. Individual children vary greatly in the amount of listening they do before they start speaking, and late starters are often long listeners. Most children will "obey" spoken instructions some time before they can speak though the word obey is hardly accurate as a description of the eager and delighted cooperation usually shown by the child. Before they can speak, many children will also ask questions by gesture and by making questioning noises. Any attempt to trace the development from the noises babies make to their first spoken words leads to considerable difficulties. It is agreed that they enjoy making noises, and that during the first few months one or two noises sort themselves out as particularly indicative of delight, distress, sociability, and so on. But since these cannot be said to show the baby’s intention to communicate they can hardly be regarded as early forms of language. It is agreed, too, that from about three months they play with sounds for enjoyment, and that by six months they are able to add new sounds to their repertoire (能发出的全部声音). This self-imitation leads on to deliberate (有意识的) imitation of sounds made or words spoken to them by other people. The problem then arises as to the point at which one can say that these imitations can be considered as speech. The third paragraph is mainly about ______.
A. the development of babies’ early forms of language
B. the difficulties of babies in learning to speak
C. babies’ strong desire to communicate
D. babies’ intention to communicate
设三阶常系数线性齐次微分方程具有特解y1=ex,y2=2xex,y3=3e-x,则该方程为______.
Modern Drugs Doctors, sixty years ago, could do little to help victims of polio. Serious cases usually ended in death. In 1955, a vaccine was developed that prevented the disease. Today, polio is no longer a major health problem. Many of the most important drugs that doctors prescribe today have been developed in the last 30 years. Modern drugs are complex, specific and powerful. People need to know more about drugs in order to use them safely. Early people discovered by accident that some of the plants growing around them seemed useful to heal sores, relieve pain, or even cure diseases. These plants were the first drugs. Now plants are still the source of some drugs. Quinine, for example, is a bitter-tasting drug used to treat the chills and fever of malaria and to reduce attacks of the disease. It is made from the bark of the cinchona tree, which grows in the Andes Mountains. The Indians of that region were the first to use the bark as a medicine. The Spanish people probably brought it to Europe in the early 1600s. Chemists learned how to get the pure drug from the bark and in 1944; it was made artificially in the laboratory. Other important drugs, such as hormones and vaccines, are obtained from animals. But most of the modern drugs come from chemical combinations worked out by research scientists. Most people never see drugs in their simple form as chemicals. Instead, they are seen as tablets, capsules or liquids that contain the drug and other ingredients. People use drugs to get different results. Some drugs attack the organism that causes a disease. They cure by killing the organism. Other drugs relieve what we call the symptoms of the disease: the headache, pain, fever or chills, and make the patient more comfortable. These constitute most over-the-counter drugs. People can get them in drug stores. The "over-the-counter drugs" are those that ______.
A. patients can get without doctors’ prescriptions
B. can relieve the symptoms of a disease
C. attack the organism that causes a disease
D. tend to be complex and powerful synthetic drugs