where s=(a+b+c+d)/2.
Heron's formula for the area of a triangle is the special case obtained by taking d=0.
... to the left of "=", that will make it a perfect square trinomial of the form x2 + 2xy + y2. Since "2xy" in this case is (b/a)x, we must have y = b/(2a), so we add the ...