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一、选择题 (本大题共 12小题 ,每小题 5分 ,共 6 0分 )1 集合M ={a ,b ,c},N ={- 1,0 ,1},映射 f :M→N满足 f(a) +f(b) +f(c) =0 ,那么映射f :M →N的个数是A 4 B 5 C 6 D 72 设e1和e2 是互相垂直的单位向量 ,且a =3e1+2e2 ,b=- 3e1+4e2 ,则a·b等于A 1 B 2 C - 1 D -
First, the multiple-choice questions (the title of a total of 12 small questions, each question 5 points, a total of 60 points) 1 set M = {a, b, c}, N = {- 1,0, 1}, mapping f: M → N satisfies f(a) + f(b) + f(c) = 0, then the number of mapping f:M → N is A 4 B 5 C 6 D 72 Let e1 and e2 be mutually perpendicular unit vectors , and a = 3e1+2e2, b=- 3e1+4e2, then a·b is equal to A 1 B 2 C - 1 D -