The basic condition for any type of triangle is:
(i) The sum of 2 sides of a triangle should be greater than the third side
(ii) The difference of any 2 sides should be less than the third side.
For a triangle to be a right angled triangle, there is an additional condition.
Pythagoras theorem: In a right angled triangle,
Hypotenuse2= Sum of squares of other 2 sides
That is, c2=a2+b2; Also note that the hypotenuse is the largest side in a right triangle.