If a line ax+by+c=0ax + by + c = 0ax+by+c=0 is a tangent to the parabola y2=4Axy^2 = 4Axy2=4Ax, then the condition is:
aA=b2caA = b^2caA=b2c
cA=ab2cA = a b^2cA=ab2
c=aA/b2c = aA / b^2c=aA/b2
b2c=aAb^2c = aAb2c=aA