※ 引述《loeooo (籃球狂)》之銘言：
: 1. how many solutions are there to the equation X1+X2+X3+X4+X5=21,
: where Xi, i=1,2,3,4,5 is a nonnegative integer such that 0<=Xi<=10
: 求整數解個數跟求非負整數解個數一樣嗎? 求整數解我會
: 但求非負要怎麼求阿?
這是用生成函數解法 每個X的生成函數(1+X^1+X^2+...+X^10)
五個就是(1+X^1+...+X^10)^5 最後求X^21的係數
: 2. There are _________ consecutive 0s at the end of the binary expansion
: of 70!
: 3. there are ________4-digit decimal telephone numbers haviing one or
: more repeated digits.
: 4. if there are five possible grades, A,B,C,D,and F, the minimum number of
: students required in a class to be sure that at least six will receive the
: same grade is ________________.
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