原式=lim (sin2x/cos2x)×tan(π/4-x),x→π/4 =（lim sin2x）×lim tan(π/4-x)/cos2x,x→π/4 = lim tan(π/4-x)/cos2x,x→π/4 = lim (sec(π/4-x))^2/(2sin2x),x→π/4 (此式由罗必达法则得） = 1/2...