Posted by Matthew on Sunday, May 2, 2010 at 9:20pm.
Use the cosine rule:
cos(A)=(b²+c²-a²)/(2bc)
When a is the hypotenuse of a right-triangle, cos(A)=0.
When a<90° cos(A)>0
When a>90° cos(A)<0
So by evaluating the cosine opposite the longest side, you can tell if the triangle is a right-triangle, acute or obtuse.
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