....................................
sin2a=2*sina*cosa;П<a<3*П/2;cosa<0;cosa=-√(1-sin²a)=-√(1-(-3/5)²)=-√(1-9/25)=-√(16/25)=-4/5;=>sin2a=2*(-3/5)*(-4/5)=24/25;=>cos2a=cos²a-sin²a;cos2a=(-4/5)²-(-3/5)²=16/25-9/25=7/25;tg2a=sin2a/cos2a=(24/25)/(7/25)=24/7=3 целых 3/7;
