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100全國高中聯招

選擇題7
n=1+22!+33!++5050!n除以50的餘數為
(A)13 (B)23 (C)29 (D)49
[解答]
nn!=(n+1)!n!
n=1+22!+33!++5050!=1+(3!2!)+(4!5!)++(51!50!)=51!1
故可知n1(mod50)
n除以50的餘數為49

綜合題1
x20+1除以(x2+1)(x44)的餘式為   
[解答]
f(x)=x20+1g(x)=x10+1,則f(x)=g(x2)
g(x)=(x+1)(x24)Q(x)+ax2+bx+c=(x+1)(x+2)(x2)Q(x)+ax2+bx+c
由餘式定理可知
g(1)=2ab+c=2g(2)=210+14a+2b+c=1025g(2)=(2)10+14a2b+c=1025a=341b=0c=339
故可知g(x)=(x+1)(x24)Q(x)+341x2339
f(x)=g(x2)=(x2+1)(x44)Q(x2)+341x4339
故所求餘式為341x4339

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