1、21²-42+1
=21²-2×21+1
=(21-1)²
=20²
=400
2、66²-6600+50²
=66²-2×66×50+50²
=(66-50)²
=16²
=256
3、(a+b)²-4ab
=a²+2ab+b²-4ab
=a²-2ab+b²
=(a-b)²
4、1/9m²n²+2/3mn+1
=(1/3mn+1)²
5、8a-4a²-4
=-4(a²-2a+1)
=-4(a-1)²