put x=y=0
f(0) = 2f(0)
f(0) = 0

put y=-x
f(0) = f(x) +f(-x)
f(x) = -f(-x)
=> f is odd

I think 且当x>0时,f（x）>0
for y> x
y= x+k ( k>0)
f(y) = f(x+k)= f(x) + f(k)
> f(x)( f(k) >0 )
f is increasing

put x=-1, y=-1
f(-2) = f(-1)+f(-1) = -4
f(1) = -f(-1) = -(-2) =2
f（x）在［－2,1］上的值域 = [-4,2]