sin^2a+2sinacosa+3cos^2a=sin^2a+cos^2a+2sinacosa+2cos^2a=1+sin2a+(1+cos2a)=sin2a+cos2a+2sina=2tan(a/2)/[1+tan^(a/2)]cosa=[1-tan^(a/2)]/[1+tan^(a/2)]所以原式=4/5+（-3/5）+2=11/5楼主的11/3是抄错了吧~...