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D( x )
9-(4*x) = 0
2*x-7 = 0
9-(4*x) = 0
9-(4*x) = 0
9-4*x = 0
9-4*x = 0 // - 9
-4*x = -9 // : -4
x = -9/(-4)
x = 9/4
2*x-7 = 0
2*x-7 = 0
2*x-7 = 0 // + 7
2*x = 7 // : 2
x = 7/2
x in (-oo:9/4) U (9/4:7/2) U (7/2:+oo)
8/(2*x-7) = 4/(9-(4*x)) // - 4/(9-(4*x))
8/(2*x-7)-(4/(9-(4*x))) = 0
8/(2*x-7)-4*(9-4*x)^-1 = 0
8/(2*x-7)-4/(9-4*x) = 0
(8*(9-4*x))/((2*x-7)*(9-4*x))+(-4*(2*x-7))/((2*x-7)*(9-4*x)) = 0
8*(9-4*x)-4*(2*x-7) = 0
100-40*x = 0
(100-40*x)/((2*x-7)*(9-4*x)) = 0
(100-40*x)/((2*x-7)*(9-4*x)) = 0 // * (2*x-7)*(9-4*x)
100-40*x = 0
100-40*x = 0 // - 100
-40*x = -100 // : -40
x = -100/(-40)
x = 5/2
x = 5/2
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