|PQ|^2=|OP-OG|^2
=(cosa-sina)^2+(2sina+2cosa)^2
=1-2sinacosa+4+8sinacosa
=5+6sinacosa
=5+3sin2a
|PQ|^2最大是5+3=8,最小是5-3=2
2^(1/2)<|PQ|<2*2^(1/2)