题目

 设为数列的前n项和，，则达到最小值时，n的值为（   ） A. 12    B. 13    C. 24    D. 25 答案：C 【解析】由an=2n−49可得数列{an}为等差数列 ∴a1=2−49=−47 Sn=×n=n2−48n=(n−24)2−242 结合二次函数的性质可得当n=24时和有最小值 故选C. 点睛：等差数列前n项和公式是，记住抛物线对称轴方程.最值一定在离对称轴最近的整数中取到.图像是过原点的抛物线上的一些离散点，由于二次函数图像的对称性，一Some wonderfully creative uses of X-rays have been in the world of art．But the most impressive one is to uncover what has been covered up．Details unfolded by an X-ray of a painting can ' t be seen any other way．For example, in the past, paintings were often repaired if small pieces of paint fell off the canvas （帆布）． These repairs cannot be seen just by looking at the painting．It takes an X-ray to discover where the repairs have been made．An X-ray of a painting is one of the best ways to tell if it was created by a famous painter or if it is a forgery（赝品）．If there is a question as to whether an old master has painted a certain painting or not, X-rays are made and compared to those of paintings that are known to be original．If the brushstrokes （笔迹） , for example, are shown to be totally different than in other work done by a certain artist, then the painting is proved to be a fake copy．Since modern paints are usually made from different materials than paints of hundreds of years ago, the image they leave on an X-ray film looks different．This is another way X-rays can be used to spot a forgery．A modem artist will try to make a forgery look hundreds of years old by painting dirty varnish on it or by using artificial means to get the varnish to look cracked．To the naked eye the forgery may look old, but when a careful study of the X-ray image is made, the deception is obvious．It was common many years ago for an artist to finish a painting, decide it wasn't any good, and paint a completely different picture on top of it．Or artists would paint over others' work —it was their version of recycling．Taking an X-ray is the only way to find out if there is a picture underneath the picture you see with your eyes．Sometimes museums will be more interested in the painting beneath than the one on top, and will restore the older one by having the top layer painstakingly removed．小题1:What is the main purpose of the article?A．To provide information to police who are looking for stolen art．B．To persuade art dealers to check for fake copies before selling art．C．To explain how modern technology can be applied in unique ways．D．To help artists restore damaged artwork to its original appearance．小题2:According to the passage，X-rays are used to ______．A．make repairs on paintings in the pastB．identify the authenticity of an artworkC．change the materials of a forgeryD．remove the recyclable paints小题3:In Paragraph 3, the word "deception" probably means ______．A．comparisonB．drawbackC．trickeryD．judgement小题4:Which is the best title for the passage?A．X-ray DetectivesB．Creative Uses of TechnologyC．Forgery StudiesD．X-ray Painting Techniques