点P（cosa,sina）在直线x+2y＝0上,即有cosa+2sina=0
tana=-1/2
则cos2a+sin2a=(cos^2a-sin^2a+2sinacosa)/(cos^2a+sin^2a)
=(1-tan^2a+2tana)/(1+tan^2a)
=(1-1/4-1)/(1+1/4)
=-1/5