和角公式
tan(A+B)=[tan A+tan B]/(1-tanA tanB)
tan(α+π/4)=-3
[tanα+tan π/4]/[1-tanαtan π/4]=-3
(1+tanα)/(1-tanα)=-3
1+tanα=-3+3tanα
2tanα=4
tanα=2
α=arctan 2