y=2-sin²a-cos²b
2y=4-2sin²a-2cos²b
=4-[1-cos2a]-[1+cos2b]
=2+cos2a-cos2b
=2-2sin(a+b)sin(a-b)
=2-2sin(5π/6)sin(150º-2b)
=2-sin(150-2b)
∴2-2y=sin(150º-2b).
∵0＜b＜90º
∴-30º＜150-2b＜150º
∴-1/2＜sin(150º-2b)≤1
即-1/2＜2-2y≤1
∴1/2≤y＜5/4
∴值域为[1/2,5/4)