tan[π/4+a]×cos 2a /2cos ²[π/4-a]=tan(π/4+a)*sin(π/2+2a)/[2cos²(π/2-(π/4+a)]=tan(π/4+a)*sin(π/2+2a)/[2sin²(π/4+a)]∵ π/2+2a=2*（π/4+a）=[sin(π/4+a)/cos(π/4+a)]*2sin(π/4+a)...