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Bài 3. a) Ta có: $~A=4-\sin^245^0+2\cos^260^0-3\cot^345^0$ $~=4-(\frac{\sqrt{2}}{2})^2+2\times (\frac{1}{2})^2-3\times 1^3$ $~=4-\frac{1}{2}+\frac{1}{2}-3$ $~=1$ b) Ta có: $~B=\tan45^0.\cos30^0.\cot30^0$ $~=1\times \frac{\sqrt{3}}{2}\times \sqrt{3}$ $~=1,5$ c) Ta có: $~C=\cos^215^0+\cos^225^0+...+\cos^275^0$ $~=(\cos^215^0+\cos^275^0)+(\cos^225^0+\cos^265^0)+(\cos^235^0+\cos^255^0)+\cos^245^0$ $~=1+1+1+\frac{1}{2}=3,5$ d) Ta có: $~D=\sin^210^0+\sin^220^0+...+\sin^280^0$ $~=(\sin^210^0+\sin^280^0)+(\sin^220^0+\sin^270^0)+...+\sin^240^0+\sin^250^0$ $~=1+1+...+1+\frac{1}{2}=4,5$