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2 ARMA model 2.1 Random simulation (1) Please use a computer to simulate the dynamic changes of the following models, and draw a 22 layout. yt=0.7yt1+t,yt=0.7yt10.49yt2+t,yt=t0.7t1,yt=0.7yt10.49yt2+t+0.7t1,tN(0,1)tN(0,1)tN(0,1)tN(0,1) Please judge whether the second model meets the requirements of stable timing, and give the basis for judgment. (2) Please make the autocorrelation two-digit graph and partial autocorrelation two-digit graph of the ARMA model above, and analyze the reasons for its changes. 2 ARMA model 2.1 Random simulation (1) Please use a computer to simulate the dynamic changes of the following models, and draw a 22 layout. yt=0.7yt1+t,yt=0.7yt10.49yt2+t,yt=t0.7t1,yt=0.7yt10.49yt2+t+0.7t1,tN(0,1)tN(0,1)tN(0,1)tN(0,1) Please judge whether the second model meets the requirements of stable timing, and give the basis for judgment. (2) Please make the autocorrelation two-digit graph and partial autocorrelation two-digit graph of the ARMA model above, and analyze the reasons for its changes