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    %mySum求和
function s=mySum(s) 
f(1)=1.5;%传染
g(1)=650;%未传染
r(1)=1;%免疫

for i=1:99
f(i+1)=0.05+f(i)*20*+g(i)*0.6*0.8
g(i+1)=g(i)-g(i)*0.6*0.8
r(i+1)=r(i)+g(i)*0.4*0.8
end
i=1:100;
plot(i,f,'ro')
figure(2)
plot(i,g,'go')
figure(3)
plot(i,r,'ko')