(1-tanα)[1-tan(3π/4-α)]
= (1-tanα)[1-(tan3π/4-tanα)/(1+tan3π/4tanα)]
= (1-tanα)[1-(-1-tanα)/(1-tanα)]
= 1-tanα-(-1-tanα)
= 1-tanα+1+tanα)
= 2
【附：两角差公式 tan(α-β)=(tanα-tanβ)/(1+tanαtanβ)】
tan3π/4= -1