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解 tanA=sinA/cosA=2 ∴sinA=2cosA ∵sin²;A+cos²;A=1 ∴sin²;A+1/4sin²;A=1 ∴5/4sin²;A=1 ∵A∈(0,π) ∴sinA>0 ∴sinA=2√5/5 選B
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