a) TH1: Nếu \(b< 0\)\(\Rightarrow a+b< a\)
TH2: Nếu \(b\ge0\)\(\Rightarrow a+b\ge a\)
b) TH1: \(a=b\)\(\Rightarrow a-b=b-a=0\)\(\Rightarrow\left(a-b\right)\left(b-a\right)=0\)
TH2: \(a\ne b\)\(\Rightarrow a-b\)và \(b-a\)đối nhau \(\Rightarrow\left(a-b\right)\left(b-a\right)< 0\)
\(\Rightarrow\left(a-b\right)\left(b-a\right)\le0\)( đpcm )