Question

Is it possible to construct a triangle with side lengths of 11 cm, 15 cm, and 21 cm? Why or why not?

Answer 1

Assume the triangle is ABC with: AB = 11 ; AC = 15 and BC = 21.
To get a triangle each side should smaller than the addition of the other sides:

AB< AC+BC (11< 15+21) For this side it's ok
AC< AB+BC (15< 11+21) For this side it's ok
BC< AB+AC ( 21<11+15. This is wrong since 26=26 thus you can't draw a triangle with such measurements