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tgα = sinα/cosα = 3 -> sinα = 3cosα
sin^2α + cos^2α = 1
9 cos^2α + cos^2α = 1
cos^2α = 1/10 -> cosα = - 1/√10 , sinα = - 3/√10
sin2α = 2sinαcosα = 2* (-1/√10)(- 3/√10) = 6/10 = 3/5