For 3 side lengths of a triangle to form a right triangle, the sum of the squares of the 2 smaller side lengths must be equal to the square of the largest side. (a² + b² = c²)
Since 1² + 2² =/= 5², (5 =/= 25)
it will not form a right triangle.
Another Explanation:
Since 5 > 1 + 2, the sum of the 2 smaller side lengths is less than the largest side. Hence it will not form a triangle => not form a right triangle.
