设f(x)连续，则
(A) xf(x2)． (B) -xf(x2)． (C) 2xf(x2)． (D) -2xf(x2)．
参考答案：A
解析： 先作变量代换将被积式中x换出来，然后再求导．
令x2-t2=u，则
则
故应选(A)．
[分析二] 排除法：取f(x)≡1，显然满足题设条件，而
由此可知，选项(B)(C)(D)都是错误的，排除(B)(C)(D)，故应选(A)．
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| Like the body, the memory improves with use. 1 the body, the memory can improve with age. For many years, doctors have been studying the way the brain 2 . We all know that the brain has two sides, the left and the right. The right side controls the senses and the left side of our brain controls our logical 3 . We call the left side the "education" side of the brain, and generally, in western countries, people have 4 this side of the brain more than the right side. Scientists believe that our brain will work more efficiently if both the right side and the left side are developed equally. In many schools today, teachers try to educate children in such a way that both sides of the brain are 5 . This can be done with logical subjects 6 math and science as well as with creative subjects such as art. Albert Einstein, the great scientist, did some of his best work when he was very old. He was 7 not only in his work, but also in creative and imaginative activities. It was because of his many different interests in life that he was able to 8 the full development of both sides of his brain. | ||||
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