∵ tan(π/4-α)= (tanπ/4-tanα)(1+tanπ/4 tanα) = (1-tanα)/(1+tanα) 分子分母同乘以cosα：= (cosα-sinα) / (cosα+sinα)∴{1 - tan^2(π/4-α) } / {1 + tan^2(π/4-α)}={1- [(cosα-sinα) / (cosα+s...