Given, sides of the triangle are 3 cm, 4 cm and 5 cm.
We have 32+42=9+16=25=52
By the converse of Pythagoras' theorem, if in a triangle, the square of one side equals the sum of squares of the remaining two sides, then the triangle is right-angled.
Thus, the given triangle is right-angled and hence, its greatest angle is 90∘