4（m+2n）²-9(2n-m)²
=（2m+4n）^2-(6n-3m)^2
=(2m+4n+6n-3m)(2m+4n-6n+3m)
=(10n-m)(5m-2n)
(a²+b²）（a²+b²-8）+16=0
设a^2+b^2=t,则t>=0
t(t-8)+16=0
t^2-8t+16=0
(t-4)^2=0
t=4
即a^2+b^2=4