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Trigonometry - Double angle formulas

  1. \(sin2 \alpha=2sin \alpha.cos\alpha\)
     
  2. \(cos2\alpha=cos^2\alpha-sin^2\alpha=1-2sin^2\alpha=2cos^2\alpha-1\)
     
  3. \(tan2\alpha= {2tan\alpha \over {1-tan^2\alpha}}={2 \over cot\alpha -tan\alpha}\)
     
  4. \(cot2\alpha={{cot^2\alpha -1} \over 2cot \alpha } = {cot\alpha - tan \alpha \over 2 }\)