∵在△ABC中,a=√5b/2,A=2B
根据正弦定理,知,a/sinA=b/sinB
∴sinA/sinB=a/b
又sinA=sin2B
=2sinBcosB
即:(2sinBcosB)/sinB=(√5b/2)/b
2cosB=√5/2
cosB=√5/4
∵在△ABC中,a=√5b/2,A=2B
根据正弦定理,知,a/sinA=b/sinB
∴sinA/sinB=a/b
又sinA=sin2B
=2sinBcosB
即:(2sinBcosB)/sinB=(√5b/2)/b
2cosB=√5/2
cosB=√5/4