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設a,b,c的次數分別為x,y,z,則有x+y+z=7且x,y,z都必須大於或等於1 這題好像沒有簡便的算灋 取x=1,就有5種情况 x=2有4種,x最大可取5(5+1+1=7) 依次類推有5+4+3+2+1=15 不知看懂否?
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