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sin2a=-8/9cos2a=+ -根號17/9(sina+cosa)^2=1+sin2a=1/9然後根據(sin2a)^2+(cos2a)^2=1你需要判斷2a的範圍cosα+sinα=-1/3=根號2sin(a+π/4)=-1/3α應該是在(3π/4,π)之間,那2α就是(3π/2,2π)之間那麼cos2a>…
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