1)tan(a-pi/4)=-2
--->(tana-1)/(1+tana)=-2
--->tana=-1/3
sin2a=2tana/[1+(tana)^2]=2(-1/3)/[1+(-1/3)^2]=(-2/3)/(10/9)=-3/5
2)3pi/4<a,b<pi--->pi/2<a-pi/4<3pi/4,3pi/2<a+b<2pi
sin(a+b)=-3/5--->cos(a+b)=4/5,tan(a+b)=-3/4
tan(b+pi/4)=tan[(a+b)-(a-pi/4)]
=[tan(a+b)-tan(a-pi/4)]/[1+tan(a+b)tan(a-pi/4)]
=[-3/4-(-2)]/[1+(-3/4)(-2)]
=(-3+8)/(4+6)
=1/2.
