因为tanα=-1/3设sina=-k cosa=3k (不管哪个为负都一样,反正sina与cosa异符号)sin2α-cos^2α/1＋cos2α=2sinacosa-cos^2a /1+2cos^2a-1=2sinacosa-cos^2a/2cos^2a=2sina-cosa/2cosa=-2k-3k/2*3k=-5/6...