Explanation:
Pythagoras must be true :
c² = a² + b²
with c being the Hypotenuse (the side opposite of the 90° angle), and it is always the longest of the 3 sides.
so, the sum of the squares of the 2 smaller numbers must be the square of the largest number.
1.3² + 2² = 1.69 + 4 = 5.69
that is NOT 3.2² = 10.24
so, this is NOT a right-angled triangle.
2.1² + 4² = 4.41 + 16 = 20.41
that is NOT 5² = 25
so, this is NOT a right-angled triangle.
5² + 12² = 25 + 144 = 169 = 13²
so, yes, this is a right-angled triangle.