a) Ta có: \(A=a\left(b+3\right)-b\left(3+b\right)\)
\(=\left(b+3\right)\left(a-b\right)\)
\(=\left(1997+3\right)\left(2003-1997\right)\)
\(=6\cdot2000=12000\)
b) Ta có: \(B=b^2-8b-c\left(8-b\right)\)
\(=b\left(b-8\right)+c\left(b-8\right)\)
\(=\left(b-8\right)\left(b+c\right)\)
\(=\left(108-8\right)\left(108-8\right)\)
\(=100\cdot100=10000\)
c) Ta có: \(C=xy\left(x+y\right)-2x-2y\)
\(=xy\left(x+y\right)-2\left(x+y\right)\)
\(=\left(x+y\right)\left(xy-2\right)\)
\(=7\cdot\left(8-2\right)=7\cdot6=42\)
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