推 privatewind :算出全部的 再去掉當一個大於10 兩個大於10 ... 04/02 22:49
→ privatewind :這是用他hint 去解，如果不想就用生成函數 04/02 22:49
find the number of solutions to the equation
x1 + x2 + x3 + x4 = 18
in which each xi, 1 <= xi <= 9
justify your answer is required
(hint: consider the inclusion and exclusion principle)
謝謝
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