Suppose the the short-run inflation-unemployment trade-off is given by the equation:
π t = π t ^e - (u-0.05) --------(1)
where π denotes inflation, π^e denotes expected inflation, u denotes the unemployment rate, and t denote time.
(解释一下t在π的右下角，我不知道怎么打出来。。= =)

Suppose also that expected inflation is given by the equation:
π t ^e= π (t-1) --------(2)         (再解释一下这里的t和t-1都在右下角)

问题如下：（主要是 第四问 ）
1 . define and calculate the natural rate of unemployment .  这个我算出来答案是0.05
2 . Discuss the meaning of the expectations formation process as described by equation (2). 这一问我知道，忽略吧。
3 . Define and calculate the sacrifice ratio in terms of unemployment
     Suppose that the economy is initially in long-run equilibrium and inflation equals 0.12. Suppose that the policy maker decides to reduce inflation to 0.02 by maintaining, each year, the unemployment rate one percentage point above the natural rate.  
    这一问我算出来的ratio 是 2.
4 . For how many years must the policy maker keep the unemploment rate above the natural rate to achieve his inflation target?
     Suppose now that after the policy has been in effect for one year, people believe that the policy maker in indeed committed to reducing inflation to 0.02. As a result, they now set their expectations of in flation according to: π t^e=0.02

这第四问不知道怎么弄啊。。。。
请教下大家 谢谢啦！~~