用万能公式,cos2A=sin(π/2+2A)=2tan[(π/4)+A]/{1+tan^2 [(π/4)+A]}=4/5 sin2A=-cos[(π/2)+2A]=-{1-tan^2 [(π/4)+A]}/{1+tan^2 [(π/4)+A]}=3/5 故sinAcosA/2+cos^2 A=sin2A/4+(1+cos2A)/2=21/20