tan (π/4+a)=(tanπ/4+tana)/(1-tanπ/4*tana)=(tana+1)/(1-tana)=3tana+1=3-3tanatana=1/2sin2a-2cos^2a=sin2a-(cos2a+1)=sin2a-coa2a-1由万能公式=2tana/[1+(tana)^2]-[1-(tana)^2]/[1+(tana)^2]-1=1/(1+1/4)-(1-1/4)/(1+1/4)-1=4/5-3/5-1=-4/5 1+tana=3/2(sin2a-2cos平方a)/(1+tana)=-8/15...