a) \(11^{1979}<11^{1980}=\left(11^3\right)^{660}=1331^{660}\)
\(37^{1320}=\left(37^2\right)^{600}=1369^{600}\)
\(1369^{660}>1331^{660}\Rightarrow11^{1979}<37^{1320}\)
b) \(1990^{10}+1990^9=1990^9\left(1990+1\right)=1990^9.1991<1991^9.1991\)
\(\Rightarrow1990^{10}+1990^9<1991^{10}\)
Vậy \(1990^{10}+1990^9<1991^{10}\)