sinx=√5/5, x属于(兀/2,3兀/2)cosx<0∴cosx=-√(1-sin²x=-2√5/5 tanx=sinx/cosx=-1/2∴tan(x-π/4)=(tanx-tanπ/4)/(1+tanxtanπ/4)=(-1/2-1)/[1+(-1/2)*1]=-3
