(21/r)-r=4

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Solution for (21/r)-r=4 equation: 


D( r )

r = 0

r = 0

r = 0

r in (-oo:0) U (0:+oo)

21/r-r = 4 // - 4

21/r-r-4 = 0

21*r^-1-1*r^1-4*r^0 = 0

(21*r^0-1*r^2-4*r^1)/(r^1) = 0 // * r^2

r^1*(21*r^0-1*r^2-4*r^1) = 0

r^1

21-r^2-4*r = 0

21-r^2-4*r = 0

DELTA = (-4)^2-(-1*4*21)

DELTA = 100

DELTA > 0

r = (100^(1/2)+4)/(-1*2) or r = (4-100^(1/2))/(-1*2)

r = -7 or r = 3

r in { -7, 3} 

r in { -7, 3 }

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