4a^2=(PF1-PF2)^2=PF1^2+PF2^2-2PF1*PF2=4c^2-8ab.
即a^2=c^2-2ab,2ab=c^2-a^2=b^2,b=2a,b^2=c^2-a^2=4a^2.
所以,c^2=5a^2,离心率e=c/a=根号5.
4a^2=(PF1-PF2)^2=PF1^2+PF2^2-2PF1*PF2=4c^2-8ab.
即a^2=c^2-2ab,2ab=c^2-a^2=b^2,b=2a,b^2=c^2-a^2=4a^2.
所以,c^2=5a^2,离心率e=c/a=根号5.