-42a в 4 степени, б, с в девятой
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X +81/x =18
(x² +81)/x =18
x² +81 =18x
x² -18x +81 =0
D =18² -4*81 =0
x1,x2 =(18+-0)/2 =9
1) D(y)=(-∞; +∞)
2) x≠0 x≠8
D(y)=(-∞; 0)U(0; 8)U(8; +∞)
3) x-7≥0
x≥7
D(y)=[7; +∞)
4) D(f)=(-∞; 1]
E(f)=(-∞; 3]