jhyap your logic of 3=4 is wrong. because from the question it is given a+b= c i.e a+b-c=0 while solving the equation you did (a+b-c)/(a+b-c) =1 which is wrong . as 0 divide by 0 can never be equal to 1. while making the eqaution you can never give such equations as it is against the rule of algebra.