Answer:
There is one theorem in triangle which states that sum of any two sides of a triangle is greater than the third side. For example, if a,b and c are three sides of a triangle, then a+b>c, b+c>a and a+c>b.
So, we will use here same concept.
Let a=5cm, b=7cm and c=12cm
a+b = 5+7 = 12cm which is equal to c.
But a+b should be greater than c.
Therefore, the triangle doesn't exist.
