已知函数f(x)对任意x，y∈R都有f(x+y)＝f(x)+f(y), 且f(2)＝4,则f(-1)＝________.在线课程-2
f(0＋0)＝f(0)＋f(0), f(0)＝0，f(-1＋1)＝f(-1)＋f(1)=0，f(1＋1)＝f(1)＋f(1)=4
f(1)＝2, f(1)＝-f(1)=-2