Question

how to know that these are lenghts of sides of the triangle
2cm 3cm 4cm​

Answer 1

this is a right angle triangle

Answer 2

To know that these sides are the sides of a ∆ Add any two sides if the sum of the two sides is greater than the third sides then they are the sides of a triangle. In this case

2+3=5

4<5

3+4=7

7>2

4+2=6

6>3

This they are the sides of a triangle.