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x in (-oo:+oo)
21*x-x^2+4*x-((6*y)/(-50))+3*y = 0
21*x-x^2+4*x+(3/25)*y+3*y = 0
25*x-x^2+78/25*y = 0
DELTA = 25^2-(-1*78/25*4*y)
DELTA = 312/25*y+625
312/25*y+625 = 0
312/25*y+625 = 0 // - 625
312/25*y = -625 // : 312/25
y = -625/312/25
y = -15625/312
DELTA = 0 <=> t_1 = -15625/312
x = -25/(-1*2) i y = -15625/312
x = 25/2 i y = -15625/312
( x = ((312/25*y+625)^(1/2)-25)/(-1*2) or x = (-(312/25*y+625)^(1/2)-25)/(-1*2) ) i y > -15625/312
( x = ((312/25*y+625)^(1/2)-25)/(-2) or x = ((312/25*y+625)^(1/2)+25)/2 ) i y > -15625/312
y+15625/312 > 0
y+15625/312 > 0 // - 15625/312
y > -15625/312
x in { 25/2, ((312/25*y+625)^(1/2)-25)/(-2), ((312/25*y+625)^(1/2)+25)/2 }
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