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D( w )
5*w-20 = 0
w-4 = 0
5*w-20 = 0
5*w-20 = 0
5*w-20 = 0 // + 20
5*w = 20 // : 5
w = 20/5
w = 4
w-4 = 0
w-4 = 0
w-4 = 0 // + 4
w = 4
w in (-oo:4) U (4:+oo)
1-7/(5*w-20) = 3/(w-4) // - 3/(w-4)
1-7/(5*w-20)-(3/(w-4)) = 0
1-7/(5*w-20)-3*(w-4)^-1 = 0
1-7/(5*w-20)-3/(w-4) = 0
(-7*(w-4))/((5*w-20)*(w-4))+(-3*(5*w-20))/((5*w-20)*(w-4))+(1*(5*w-20)*(w-4))/((5*w-20)*(w-4)) = 0
1*(5*w-20)*(w-4)-7*(w-4)-3*(5*w-20) = 0
5*w^2-22*w-40*w+80+88 = 0
5*w^2-62*w+168 = 0
5*w^2-62*w+168 = 0
5*w^2-62*w+168 = 0
DELTA = (-62)^2-(4*5*168)
DELTA = 484
DELTA > 0
w = (484^(1/2)+62)/(2*5) or w = (62-484^(1/2))/(2*5)
w = 42/5 or w = 4
(w-4)*(w-42/5) = 0
((w-4)*(w-42/5))/((5*w-20)*(w-4)) = 0
((w-4)*(w-42/5))/((5*w-20)*(w-4)) = 0 // * (5*w-20)*(w-4)
(w-4)*(w-42/5) = 0
( w-4 )
w-4 = 0 // + 4
w = 4
( w-42/5 )
w-42/5 = 0 // + 42/5
w = 42/5
w in { 4}
w = 42/5
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