x²+4x/3=200
x²+4x/3+4/9=200+4/9
(x+2/3)² = 1804/9
(x+2/3)² = （2√451/3）²
x+2/3=2√451/3,或者,x+2/3=-2√451/3
x = （-2±2√451）/3