5x=0.5x^2-200
0.5x^2-5x-200=0
x^2-10x-400=0
x^2-10x=400
x^2-10x+25=425
(x-5)^2=425
x-5=±5√17
x=±5√17+5
x1=5√17+5.x2=-√17+5