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D( x )
x-7 = 0
x-8 = 0
x-7 = 0
x-7 = 0
x-7 = 0 // + 7
x = 7
x-8 = 0
x-8 = 0
x-8 = 0 // + 8
x = 8
x in (-oo:7) U (7:8) U (8:+oo)
1/(x-7)+1/(x-8) = 15/56 // - 15/56
1/(x-7)+1/(x-8)-(15/56) = 0
1/(x-7)+1/(x-8)-15/56 = 0
(1*56*(x-8))/(56*(x-7)*(x-8))+(1*56*(x-7))/(56*(x-7)*(x-8))+(-15*(x-7)*(x-8))/(56*(x-7)*(x-8)) = 0
1*56*(x-8)+1*56*(x-7)-15*(x-7)*(x-8) = 0
112*x-15*x^2+225*x-840-840 = 0
337*x-15*x^2-1680 = 0
337*x-15*x^2-1680 = 0
337*x-15*x^2-1680 = 0
DELTA = 337^2-(-1680*(-15)*4)
DELTA = 12769
DELTA > 0
x = (12769^(1/2)-337)/(-15*2) or x = (-12769^(1/2)-337)/(-15*2)
x = 112/15 or x = 15
(x-112/15)*(x-15) = 0
((x-112/15)*(x-15))/(56*(x-7)*(x-8)) = 0
((x-112/15)*(x-15))/(56*(x-7)*(x-8)) = 0 // * 56*(x-7)*(x-8)
(x-112/15)*(x-15) = 0
( x-112/15 )
x-112/15 = 0 // + 112/15
x = 112/15
( x-15 )
x-15 = 0 // + 15
x = 15
x in { 112/15, 15 }
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