sin^4 X+cos^4 X=5/9,
sin^4 X+cos^4 X+2sin^2Xcos^2X-2sin^2Xcos^2X=5/9,
(sin^2X+cos^2X)^2-2sin^2Xcos^2X=5/9
1-1/2*(sin2x)^2=5/9
(sin2x)^2=8/9
因为X为第三象限的角,即2kπ+π