2) equilateral triangle. The original deformation is sinA/cosA=sInB/cosB=sinC/cosC, that is, tanA=tanB=tanC,

In the triangle, tanA+tanB+tanC=tanA*tanB*tanC, so 3 3 3tanA=tanA^3, tanA=sqrt(3), A=π/3, similarly, B=C=π/3.

3) obtuse triangle. A * b = | a | * | b | * cos<a, b> knows that cos < a, b>& gt0 means that the complement angle of ∠ABC is acute, so ∠ABC is obtuse.

4)c =((- 1/5)* sqrt( 10)，(3/5)*sqrt( 10))。 Let c=k( (a/|a|)+(c/|c|)) (vector bisector formula), then c = k ((0,1)/1)+((-3,4)/5)) = k.

And |c| = 2, so k = sqrt( 10)/3. So c = ((-1/5) * sqrt (10), (3/5) * sqrt (10)).