Đặt A=\(\left(x-5\right)\left(y+40\right)\)
\(a,x=5;y=1000\)
\(A=\left(5-5\right)\left(1000+40\right)\)
\(A=0.1040\)
\(A=0\)
\(b,x=-10;y=15\)
\(A=\left(-10-5\right)\left(15+40\right)\)
\(A=\left(-15\right).55\)
\(A=-825\)
\(c,x=16;y=-50\)
\(A=\left(16-5\right)\left(-50+40\right)\)
\(A=11.\left(-10\right)\)
\(A=-110\)