Co 3 cach lam :
Cach 1:Do goc xOy va goc yOz => goc xOy + goc yOz = goc xOz
Cach 2:Do goc xOy va goc xOz => goc xOz - goc xOy = goc yOz
Cach 3:Do goc yOz va goc xOz => goc xOz - goc yOz = goc xOy
Trong day , minh ghi co chu "goc" thi ban nho thay the bang ki hieu nghe!