答:
f(x+y)=f(x)+f(y) 已知f(2)=1
f(1+1)=f(1)+f(1)=2f(1)=1,f(1)=1/2
f(3)=f(2+1)
=f(2)+f(1)
=1+1/2
=3/2
f(3)=3/2