OJ Board

Finally , I get it caused by the precision.It may help you.

I hvae got following info

double ang = acos((ab * ab + ac * ac - bc * bc) / (2.0 * ab * ac)); //AC

double ang = acos(a*b / (a.norm()*b.norm())); //WA

ab,ac,bc are the side of the triangle
a.norm() is the distance between a and orgin (0,0)

Have somebody know why using dot product to calc the ...