√(9-4√5)
=√[(√5)^2-2*2*√5+2^2]
=√(√5-2)^2
=√5-2
√(x^2+1/x^2-2)
=√[x^2-2*x*1/x+(1/x)^2]
=√(x-1/x)^2
=|x-1/x|不明白√(9-4√5)真么过渡到这一步√[(√5)^2-2*2*√5+2^2]能说明下么？万分感激~~~~