sin^4b+cos^4b+2sin²bcos²b=5/9+2sin²bcos²b
(sin²b+cos²b)²=5/9+1/2sin²2b
1=5/9+1/2sin²2b
所以
1/2sin²2b=1-5/9
sin²2b=8/9
sin2b=±2√2/3