题目

More than one million children in the United States do not go to school.Instead, they learn at home.Most often, their parents are their teachers.Educational companies, libraries and the Internet provide many families with teaching materials.        Parents choose home schooling for several reasons.Some choose it because of their religious beliefs.Others say it provides more time for the family to be together.They say the home offers a better place for learning.Some parents believe home schooling avoids learning.Some parents believe home schooling avoids problems of nati用f(n)表示自然数n的各位数字的和,如f(20)=2+0=2,f(2009)=2+0+0+9=11,对任意的自然数n,都有n+f(n)≠x,则满足这个条件的最大的两位数x的值为 .
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