已知x∈(-π/2,0),cosx=4/5
所以sinx=-√[1-(4/5)²]=-3/5
所以tanx=sinx/cosx=(-3/5)/(4/5)=-3/4
所以tan(x-π/4)=[tanx-tan(π/4)]/[1+tanx*tan(π/4)]=(-3/4-1)/[1+(-3/4)*1]=-7