tana=tan[π/4-(π/4-a)]=[tan(π/4)-tan(π/4-a)]/[1+tan(π/4)tan(π/4-a]=(1+1/2)/(1-1/2)=3∴ cos2a=cos²a-sin²a=(cos²a-sin²a)/(cos²a+sin²a)同除以cos²a=(1-tan²a)/(...