a: \(a^2+b^2=\left(a+b\right)^2-2ab=25-12=13\)
b: \(A=a^3-b^3=\left(a-b\right)^3+3ab\left(a-b\right)\)
\(\left(a-b\right)^2=\left(a+b\right)^2-4ab=\left(-5\right)^2-4\cdot6=1\)
=>a-b=1 hoặc a-b=-1
TH1: a-b=1
\(A=1^3+3\cdot6\cdot1=19\)
TH2: a-b=-1
\(A=\left(-1\right)^3+3\cdot6\cdot\left(-1\right)=-1-18=-19\)