a=4/(√5-1)
=4(√5+1)/(√5-1)(√5+1)
=√5+1
a^3-2a^2-4a
=a(a^2-2a-4)
= a〔(a-1)^2-5 〕
=(√5+1)〔(√5+1-1)^2-5 〕
=(√5+1)(5-5 )
=0