cosα-sinα=3√5/5 平方得1-sin2a=9/5sin2a=-4/5 17π/12＜α＜7π/4cos2a=-3/5tan（π/4+α）=(1+tana)/(1-tana)=(sina+cosa)/(cosa-sina)=(sina+cosa)^2/(cos^2a-sin^2a)=(1+sin2a)/cos2a=(1/5)/(-3/5)=-1/3...