# Answer to Question #17920 in Trigonometry for Marica

Question #17920

proof identity (1-tanA)/(1+tanA)=(cos2A)/(1+sin2A)

Expert's answer

(1-tanA)/(1+tanA)=(1-sinA/cosA)/(1+sinA/cosA)=(cosA-sinA)/(cosA+sinA)=(cos^2A-sin^2A)/(cosA+sina)^2=cos2A/(sin^2A+cos^2A+2sinAcosA

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