f(x)=-1 khi x<0. Tính f(-2), f(-1), f(0), f(2) ,f(5) f(x)=0 khi x=0.tính f(-2), f(-1), f(0), f(2) ,f(5) f(x)=1.tính f(-2), f(-1), f(0), f(2) ,f(5)