tan(π/4+α)=1/2∴ [tan(π/4)+tanα]/[1-tan(π/4)tanα]=1/2∴ (1+tanα)/(1-tanα)=1/2∴ 2+2tanα=1-tanα∴ tanα=-1/3∴ tan2α=2tanα/(1-tan²α)=2*(-1/3)/[1-(-1/3)²]=(-2/3)/(8/9)=-3/4求sin2阿～-cos方阿～除以1＋cos2阿的值sin2阿～-cos方阿～除以1＋cos2阿=(sin²α-cos²α)/(2cos²α+sin²α) 分子分母同时除以cos²α=(tan²α-1)/(2+tan²α)=(-8/9)/(19/9(=-8/19