1/cos2x+tan2x=(1+sin2x)/cos2x=(sin²x+cos²x+2sinxcosx)/(cos²x-sin²x)=(sinx+cosx)²/[(cosx+sinx)(cosx-sinx)]=(sinx+cosx)/(cosx-sinx)=(tanx+1)/(1-tanx)=2008sinx+siny=sin225°=sin(...在△ABC中,已知向量AB=(0,2),BC=(3,4),则△ABC的边AB与BC所成角的余弦值向量AB*BC=0-2×4=-8而|AB|=2，|BC|=√(9+16)=5所以cos=-8/(2×5)=-4/5而△ABC的边AB与BC所成角a=180°-所以cosa=-cos=4/5