Question

集合A={1,2,3},b={3,4},从A到B的映射满足f(3)=3,则这样的映射共有多少个

答案上是4，我想要步骤 谢谢

Answer 1

f(1)=3，f(2)=3，f(3)=3
f(1)=3，f(2)=4，f(3)=3
f(1)=4，f(2)=3，f(3)=3
f(1)=4，f(2)=4，f(3)=3
满足要求的映射有四个

Answer 2

A∪B=1234
A∩B=3

f(3)=3