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线性方程组AX=B(A为m×n矩阵)有解的充要条件是（ ）
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若方程组AX=B中，方程的个数小于未知量的个数，则有()．
Rules are important for children. They can help to keep them safe, as well as teach them how to well in school, society and at home.Kindergarten is a good place to teach children some rules, where they can learn the rules in their everyday life. They need to line up when they want to get the water. They shouldn’t each other when go to the toilet. They shouldn’t speak dirty words. They need to say “thank you” when others help them. They need to love the flowers and never them up.But sometimes teachers may find it’s not easy to make little children the rules. Here’s what you can try:Make children know the importance of rules through their own experience. When they really the results of not obeying the rules, they may want to follow them next time.Let children make the rules themselves. Sometimes teachers can try to give children the chance to make their own rules. Then if somebody the rules, there must be somebody else tells him not to do that.Design some activities to help children develop . Some children are just too young to remember the rules. Teachers should remind them from time to time and offer praise when the children follow the rules.Get parents’ help. Tell parents the importance of having their childrenthe social rules and ask them to make good examples for children. You can ask parents to make some rules with their children and try to follow the rules in their life.规则对孩子们来说很重要。规则既可以保证孩子们的安全，又可以教会他们在学校、社会和家里如何表现。幼儿园是教会孩子们基本规则的好地方，在这里孩子们在日常生活中学习规矩。打水时要排好队，上厕所时不要互相推搡，不能说脏话，当别人帮忙时需要说“谢谢”，要爱护花草不要摘花。但有时，老师们发现要让小孩子遵守规则并不容易。你可以试试这些方法：让孩子通过自身的体验明白规则的重要性。当孩子真正体验到不遵守规则的后果时，下次他们也许就会遵守了。让孩子自己制定规则。有时，老师可以给孩子机会让他们自己制定规则，那么如果有人破坏了规则，就会有人告诉他不要那样做。设计一些活动帮助孩子提高自我控制能力。有些太小记不住规则，老师需要不断地提醒，并且在孩子遵守规则时要及时表扬。得到父母的帮助。告诉父母让孩子遵守社会规则的重要性，并让父母为孩子树立好的榜样。你可以要求父母和孩子一起制定规则并在生活中遵守它。
在三视图中,主视图反映物体的( )。
在三视图中,俯视图反映物体的( )。
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