a) $cot\left(x\right)=1$

 b) $cot\left(x\right)=-1$

 c) $cot\left(x\right)=0$

Lời giải chi tiết:

a) $cot\left(x\right)=1\iff cot\left(x\right)=cot\left(\frac{\mathrm{\pi }}{4}\right)\iff x=\frac{\mathrm{\pi }}{4}+k\mathrm{\pi },\mathrm{k}\in \mathrm{\mathbb{Z}}$

b) $cot\left(x\right)=-1\iff cot\left(x\right)=cot\left(-\frac{\mathrm{\pi }}{4}\right)\iff x=-\frac{\mathrm{\pi }}{4}+k\mathrm{\pi },\mathrm{k}\in \mathrm{\mathbb{Z}}$

c) $cot\left(x\right)=0\iff cot\left(x\right)=cot\left(\frac{\mathrm{\pi }}{2}\right)\iff x=\frac{\mathrm{\pi }}{2}+k\mathrm{\pi },\mathrm{k}\in \mathrm{\mathbb{Z}}$