tan(π/4-x)=1/2
所以tan[2(π/4-x)]=2tan(π/4-x)/{1-[tan(π/4-x)]^2}
=2×(1/2)/[1-(1/2)^2]
=1/(3/4)
=4/3
即tan(π/2-2x)=4/3,所以cot2x=tan(π/2-2x)=4/3