cos(x-π/4)=cos(π/4-x)=sin[π/2-(π/4-x)]=sin(x+π/4)=4/5sin²(x-π/4)+cos²(x-π/4)=0所以sin(x-π/4)=±3/5所以tan(x-π/4)=sin(x-π/4)/cpos(x-π/4)=±3/4
