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BP为( )。
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a.c为( )。
The ups and downs of life may seem to have no predictable plan. But scientists now know there are very definite patterns that ahnost all people share. Even if you’ve passed some of your "prime", you still have other prime years to experience in the future. When are you smartest From 18 to 25, according to I. Q. scores; but you’re wiser and more experienced with increasing age. You’re sharpest in your 20’s. But your I.Q. for other tasks climbs. Your vocabulary at age 45, for example, is three times as great as when you graduated from college. At 60, your brain possesses almost four times as much information as it did at age 21. When are you happiest You have the best physical sense of yourself from 15 to 24; the best professional sense from 40 to44. Before age 24, we believe that our happiest years are yet to come; over 30, we believe that they’re behind as. A National Health Survey agrees: After age 30, we become more realistic and do not view happiness as a goal in itself. If we maintain our health, achieve professional and emotional goals, happiness will follow. When are you most creative Generally between 30 and 39, but the peak varies with different professions. Mozart wrote a symphony by age eight, and Mendelssohn composed his best known work at 17, but most of the great music was written by men between 33 and 44. Though the peak in most fields comes early, creative people continue to produce quality work throughout their lives. For the "well-conditioned mind", there is no upper limit. There are many ways to take happiness with us in our lives EXCEPT FOR ______.
Many years ago there was a huge oil refinery fire. Flames shot hundreds of feet into the air. The sky was thick with black smoke. The heat was so intense that firefighters had to park their trucks a block away and wait for the heat to die down before they could begin to fight the fire. However, it was about to rage out of control. Then, all of a sudden, from several blocks away came a fire truck racing down the street. With its brakes screeching, it hit the curb in front of the fire. The firefighters jumped out and began to battle the blaze. All the firefighters who were parked a block away saw this, and they jumped into their trucks, drove down the block and began to fight the fire, too. As a result of that cooperative effort, they were just barely able to bring the fire under control. The people who saw this teamwork thought: "My goodness, the man who drove that lead fire truck—what an act of bravery!" They decided to give him a special award to recognize him for his bravery in leading the charge. At the ceremony the mayor said, " Captain, we want to honour you for a fantastic act of bravery. You prevented the loss of property, perhaps even the loss of life. If there is one special thing you could have — what would it be" Without hesitation, the captain replied, "Your Honour, a new set of brakes would be nice!\ Why did the fire truck lead the charge since other firefighters parked their trucks a block away
1.设A为m×n阶矩阵，证明：r(ATA)=r(A)；
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