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x in (-oo:+oo)
2*cos(x/3) = 0
cos(x/3) = 0
cos(x/3) = 0 <=> x/3 = pi*k_1+pi/2 i k_1 należy do I
t_1 = pi*k_1+pi/2
x/3-t_1 = 0
1/3*x-t_1 = 0 // + t_1
1/3*x = t_1 // : 1/3
x = t_1/1/3
x = 3*t_1
x = 3*pi*k_1+pi/2 i k_1 należy do I
2*cos(x/3) = 0 <=> 2 = 0 or 2*cos(x/3) = 0 <=> cos(x/3) = 0
x = 3*pi*k_1+pi/2
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