tanπ/4=1tan(π/4+a)=(tanπ/4+tana)/(1-tanπ/4tana)=(1+tana)/(1-tana)=3tana=1/2sina/cosa=1/2cosa=2sina代入恒等式sin²a+cos²a=1sin²a=1/5cos²a=4/5sin2a=2sinacosa=2sina*2sina=4sin²...