tan(α+π/4）=3/2
(tanπ/4+tanα)/(1-tanαtanπ/4)=3/2
(1+tanα)/(1-tanα)=3/2
2+2tanα=3-3tanα
5tanα=1
tanα=1/5
2(sinα)^2-(cosα)^2=(2(sinα)^2-(cosα)^2)/((sinα)^2+(cosα)^2)=(2(tanα)^2-1)/((tanα)^2+1)
=(2/25-1)/(1/25+1)=-(23/25)/(26/25)=-23/26