A triangle is valid if sum of its two sides is greather than the third side. If three sides are a, b and c, then three conditions should be met.
10. 5cm, 10cm, 2cm
[tex]\begin{gathered} 5+10>2 \\ 15>2\text{ -> False} \end{gathered}[/tex]This cannot be a triangle.
11. Since it has all its angles equals, that means it is a equilateral triangle.
12. 7cm, 7cm, and 15cm
[tex]\begin{gathered} 7+7>15 \\ 14>15\text{ -> False} \end{gathered}[/tex]It cannot be a triangle.
