It is known that Tan (a pie / 4) = 1 / 2 and - Pie / 2

It is known that Tan (a pie / 4) = 1 / 2 and - Pie / 2

tan(α+π/4)=(tanα+tan(π/4))/(1-tanα*tan(π/4))=(tanα+1)/(1-tanα)=1/2
=> tanα=-1/3
-π/2