已知-∏/6≤B≤∏/4,3sin^2 a-2sin^2 b=2sina,求sin^2 b-1/2sina的最小值.
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已知-∏/6≤B≤∏/4,3sin^2 a-2sin^2 b=2sina,求sin^2 b-1/2sina的最小值.
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![已知-∏/6≤B≤∏/4,3sin^2 a-2sin^2 b=2sina,求sin^2 b-1/2sina的最小值.](/uploads/image/z/19210984-16-4.jpg?t=%E5%B7%B2%E7%9F%A5-%E2%88%8F%2F6%E2%89%A4B%E2%89%A4%E2%88%8F%2F4%2C3sin%5E2+a-2sin%5E2+b%3D2sina%2C%E6%B1%82sin%5E2+b-1%2F2sina%E7%9A%84%E6%9C%80%E5%B0%8F%E5%80%BC.)
∵-π/6≤b≤π/4
∴-1/2≤sinb≤ √2/2
3sin² a-2sin² b=2sina
∴sin²b=3sin²a /2 - sina
=(3/2)(sina-1/3)²-1/6∈[-1/2,√2/2]
∴2/3≤sina≤1或-1/3≤sina≤0
sin² b-1/2sina=3sin²a /2 - sina-1/2sina
=3sin²a/2-3sina/2
=3/2(sina-1/2)²-3/8
∴-1/3≤3/2(sina-1/2)²-3/8≤0或0≤3/2(sina-1/2)²-3/8≤2/3
即:-1/3≤sin² b-1/2sina≤0或0≤sin² b-1/2sina≤2/3
∴sin² b-1/2sina的最小值为-1/3.
∴-1/2≤sinb≤ √2/2
3sin² a-2sin² b=2sina
∴sin²b=3sin²a /2 - sina
=(3/2)(sina-1/3)²-1/6∈[-1/2,√2/2]
∴2/3≤sina≤1或-1/3≤sina≤0
sin² b-1/2sina=3sin²a /2 - sina-1/2sina
=3sin²a/2-3sina/2
=3/2(sina-1/2)²-3/8
∴-1/3≤3/2(sina-1/2)²-3/8≤0或0≤3/2(sina-1/2)²-3/8≤2/3
即:-1/3≤sin² b-1/2sina≤0或0≤sin² b-1/2sina≤2/3
∴sin² b-1/2sina的最小值为-1/3.
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