The complex conjugate of the given points are in arg and plane A(1−2i),B+(2+3i),C(3+4i) So, a=BC =√1+1 =√2 And, b=CA =√4+36 =√40 And, c=AB =√1+25 =√26 This implies, cos‌B=
a2+c2−b2
2ac
=
2+26−40
2√2√26
=−
12
4√13
=−
3
√13
<0 So, the points A, B, C represent vertices of an obtuse triangle.