 
       
      
    4. BE and CF are two equal altitudes of a triangle ABC. Using RHS congruence rule, prove that the triangle ABC is isosceles.
 
                
                  Using the given conditions, consider    and     
,
(i)                   (Right angle)
(ii)                                  (Common in both the triangles)
(iii)                                (Given that altitudes are of the same length. )
Thus by RHS axiom, we can say that  :                
Hence by c.p.c.t.,     
And thus        (sides opposite to equal angles are also equal). Thus ABC is an isosceles triangle.
 
                