M={-1,0,1},N={5,6,7,8,9}
f:M->N
x+f(x)+xf(x) is odd number
x =0
x+f(x)+xf(x) = f(0)
=>f(0) is odd number
f(0) = 5 or 7 or 9
x=-1
x+f(x)+xf(x) = -1 is odd number
=>f(-1) = 5 or 6 or 7 or 8 or 9
x=1
x+f(x)+xf(x) = 1+2f(1) is odd number
=> f(1) = 5 or 6 or 7 or 8 or 9
求這樣的映射f的個數(shù)
= 3(5)^2
=75什么叫 is odd number是奇數(shù)　　x+f(x)+xf(x) = f(0) 這是為什么？　　=>f(-1) = 5 or 6 or 7 or 8 or 9怎么推的？　　=> f(1) = 5 or 6 or 7 or 8 or 9怎么推得？　　求這樣的映射f的個數(shù)= 3(5)^2=75為什么？x=0x+f(x)+xf(x) = 0+f(0) +0f(0) = f(0)x=-1x+f(x)+xf(x) =-1是奇數(shù)=>f(-1) 可以是任何屬于N的數(shù) f(-1) = 5 or 6 or 7 or 8 or 9x=1x+f(x)+xf(x) = 1+2f(1) 是奇數(shù)=>f(-1) 可以是任何屬于N的數(shù)=> f(1) = 5 or 6 or 7 or 8 or 9f(-1) 可以是任何屬于N的數(shù) ？為什么f（0）怎么推求這樣的映射f的個數(shù)= 3(5)^2=75為什么？