A^2= 1/2(s-a)(s-b)(s-c)
OK
multiply both sides by 2
2A^2 = (s-a)(s-b)(s-c)
divide both sides by (s-a)(s-b)
[2A^2]/[(s-a)(s-b)] = s-c
Add c to both sides and subtract the expression on the left side from both sides
c = s - [2A^2]/[(s-a)(s-b)]
We could get a common denominator here, but the final result would be a little messy...this is good enough !!!
