6: \(37^2+2\cdot37\cdot13+13^2\)
\(=\left(37+13\right)^2\)
\(=50^2=2500\)
7: \(51,7^2-2\cdot51,7\cdot31,7+31,7^2\)
\(=\left(51,7-31,7\right)^2\)
\(=20^2=400\)
8: \(20,1\cdot19,9\)
\(=\left(20+0,1\right)\left(20-0,1\right)\)
\(=20^2-0,1^2=400-0,01=399,99\)



