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D( x )
cos(6*x) = 0
cos(6*x) = 0
cos(6*x) = 0
cos(6*x) = 0 <=> 6*x = pi*k_1+pi/2 i k_1 należy do I
t_1 = pi*k_1+pi/2
6*x-t_1 = 0
6*x-t_1 = 0 // + t_1
6*x = t_1 // : 6
x = t_1/6
x = pi*k_1+pi/2/6 i k_1 należy do I
x in {( -oo : +oo ) / < pi*k_1+pi/2/6 : pi*k_1+pi/2/6 >} i k_1 -> {I}
1/cos(6*x) = 0
1/cos(6*x) = 0 // * cos(6*x)
1 = 0
x belongs to the empty set
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