The third side of the triangle is 26.
Explanation:  
Given: 
Two sides of the isosceles  triangle are 26 and 10.  
To Find:
The possible value of the third side
Solution:
Definition of Isosceles  triangle:
An isosceles triangle is a triangle with (at least) two equal sides. Isosceles triangle  has both two equal sides and two equal angles.
We also know the property of the triangle that  
Sum of two sides of triangle > third side of the triangle  
Case1. When third side is 10
Thee third side can’t be equal to 10 because it does not follow the rule of triangle  
10+10>26………which is  not possible
So the third side of the isosceles triangle is 26.