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D( x )
21*x-63 = 0
x-3 = 0
21*x-63 = 0
21*x-63 = 0
21*x-63 = 0 // + 63
21*x = 63 // : 21
x = 63/21
x = 3
x-3 = 0
x-3 = 0
x-3 = 0 // + 3
x = 3
x in (-oo:3) U (3:+oo)
22/(21*x-63)+11/(x-3) = 0
(22*(x-3))/((21*x-63)*(x-3))+(11*(21*x-63))/((21*x-63)*(x-3)) = 0
22*(x-3)+11*(21*x-63) = 0
253*x-759 = 0
(253*x-759)/((21*x-63)*(x-3)) = 0
(253*x-759)/((21*x-63)*(x-3)) = 0 // * (21*x-63)*(x-3)
253*x-759 = 0
253*x-759 = 0 // + 759
253*x = 759 // : 253
x = 759/253
x = 3
x in { 3}
x belongs to the empty set
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