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若向量,与都是非零向量,则“++=(零向量)”是“∥(+)”的( ) A.充分而...
若向量
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,
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与
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都是非零向量,则“
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+
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+
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=
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(零向量)”是“
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∥(
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+
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)”的( )
A.充分而不必要条件
B.必要而不充分条件
C.充分必要条件
D.既不充分与不必要条件
考点分析:
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已知
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=1-ni,其中m,n是实数,i是虚数单位,则m+ni的虚部为( )
A.1
B.2
C.i
D.2i
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命题:“若a
2+b
2=0(a,b∈R),则a=b=0”的逆否命题是( )
A.若a≠b≠0(a,b∈R),则a
2+b
2≠0
B.若a=b≠0(a,b∈R),则a
2+b
2≠0
C.若a≠0且b≠0(a,b∈R),则a
2+b
2≠0
D.若a≠0或b≠0(a,b∈R),则a
2+b
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已知:f
n(x)=a
1x+a
2x
2+…+a
nx
n,f
n(-1)=(-1)
n•n,n=1,2,3,…
(I)求a
1、a
2、a
3;
(II)求数列{a
n}的通项公式;
(II)求证:
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.
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学校要建一个面积为392m
2的长方形游泳池,并且在四周要修建出宽为2m和4m的小路(如图所示).问游泳池的长和宽分别为多少米时,占地面积最小?并求出占地面积的最小值.
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设△ABC的内角A,B,C所对的边分别为a,b,c,已知
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.
(Ⅰ)求角C的大小;
(Ⅱ)求△ABC的面积.
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