tan (π/4+a)
=(tanπ/4+tana)/(1-tanπ/4*tana)
=(tana+1)/(1-tana)=3
tana+1=3-3tana
tana=1/2
sin2a-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