f(x+1)
=(X+1)-(X+1)^2
=X+1-(X^2+2X+1)
=X+1-X^2-2X-1
=-X^2-X
f(x+1)–f(x)
=-X^2-X-(X-X^2)
=-X^2-X-X+X^2
=-2X
不懂的欢迎追问,